That was what I grew up with. Lived in house from 1985ish until 2003.
I grew up in an apartment building built in the twenties (and have never lived in a house in my life), but our apartment was unusual in that it had three bathrooms (for a family of six). One was like those mentioned by OldGuy and aceplace57 – it had an entrance from a bedroom, and another into the hallway. The other two were what I think is meant by a master bathroom – they were full bathrooms, attached to a bedroom, with no other entrance.
Most the typical ranch homes here (Santa Clara County, CA) that were built in the late 50s and early 60s did not have a bath adjoined to the master bedroom. They are typically 3BR 2BA, but the 2 baths are both off the hall. One might be used exclusively by the parents, but you had to go into the hallway to access it.
I do remember the ranch style home my parents bought in 1967 on the East Coast in a fairly nice Middle Class neighborhood did have a master bath. I even think the kids’ bath had a 2-sink vanity!
Fancy can be either a verb or an adjective. Maybe we call it “en suite” because that’s what hotels call it, and that’s where you’re most likely to find one. Or maybe because the French were the last to invade, we shared monarchs for a while, and they’re only about 20 miles away, so we actually have a fair bit of French lying about here and there. Or it could be because it’s a shorter, more convenient expression than “master bathroom”, leaves no room for confusion (how many doors? From where now?), and needn’t refer only to the master bedroom having its own exclusive facilities (i.e. any bedroom can be en suite, not just the main one). Why use more words than necessary? Strange people, you Americans. 
Anyway. They’re becoming more common here, but not ubiquitous by any means. Most houses and flats in my experience do not have them. I’ve lived in 21 homes, two of which had en suite bathrooms.
When my mother had our house built in 1953, the big bedroom had its own bathroom, with a shower rather than a tub. Eventually, my brother got that room–since he was The Boy. It was just one of the builder’s options & we were definitely not rich. She always wished she’d had the main bathroom built larger.
Another early “luxury” feature: In the late 60’s, I lived in a Houston apartment that had two walk-in closets. One smallish & one large enough to keep the ironing board set up inside it. It was actually a duplex–built in the 1940’s, I’d guess.
The first house my parents purchased has three bedrooms, one bath. This was early 60s. In the late 70s, the decided to remodel and turn the attic into four bedrooms. Even then there was no master bath. Just one that all the bedrooms shared and one downstairs.
As others have noted, the term “master bathroom” seems to be mostly an Americanism, and in the UK and Canada the term “ensuite” is used to denote a bathroom exclusively attached to the master bedroom (or technically, a bathroom exclusively attached to any bedroom). I’ve always heard it as “ensuite” and “master bathroom” sounds a little odd to my ears because “master” implies “main” or “central”, so what do you call the main bathroom that everybody else uses?
Anyway in North America the current architectural trend for quite some decades now has been that a house is not a house unless it has a master bedroom large enough to hangar a 747, connected to a correspondingly gigantic ensuite and a walk-in closet. This all seems to have started around the time that women got seriously involved in making house-buying decisions. A guy would just need a place to put a bed, and focus on the important stuff like the home theatre room and the garage. 
The house I mostly grew up was built in the late 1960s in a subdivision neighborhood in northeastern Wisconsin that was mostly filled in around that same time, in fact there are a number of houses that are identical to the one I grew up save for outside trim touches, if you went in the front door the layout was all basically identical. (Living room on the right, steps upstairs to the left, hallway straight ahead to the kitchen, .5 bath off the hall, family room off the kitchen eating area to the left, dining room off the kitchen kitchen area to the right. Sorted.)
The original design lacked the master bath and my house did as well. It was a 2 story 1.5 bath house, with the single full bath upstairs with the four bedrooms, the master being only slightly larger than the “kids” bedrooms.
But by the mid 1980s a number of the houses in the neighborhood that had the same design and that I got to see inside of already had modified the upstairs to put a master bath by cutting the number of bedrooms upstairs down to three (or leaving a very small vestigial 4th bedroom). These people had just one or two kids, so no big whoop. The third bedroom becomes an office/shared room, as people got video game systems and computers, they tended to end up in that room.
Master bathrooms weren’t unknown in the neighborhood, the next door neighbors had a 1.5 story house, somewhat of a unique design within the neighborhood. There were three bedrooms upstairs for their 2 kids, the master had its own bath. When the Mom & Dad had a kid late in life, their oldest ended up with a bedroom carved out of the corner of the basement. We neighborhood kids were pretty jealous of that sweet setup.
My parents sold the house in 1991 with it still having the 1.5 bath/4 bedroom layout. I see looking at the neighborhood in Google Maps nowadays the current owner (still the people who bought the house in 1991) bumped out the back of the house, so I presume the house finally got its master bath and a bigger kitchen sometime in the 90s or early 2000s.
I’ve owned two homes built in the early 70s and lived in one built in the early 70s. All had master bathrooms, though all of them only had a shower and toilet, no bathtub. I would say in the Maryland suburbs it’s quite common to have one, at least in homes built after the early 70s, if not earlier.
Master as in owner or boss, and denoted some sort of exclusivity and perhaps some luxury. The main bathroom is exactly that the main (common) bathroom.