You have to bend your knees. Straighten your legs, and now your shoulders and chest are out of the water. Why is that? Anti-drowning safety feature? Water-conservation feature? Or what?
Baths are for kids
Weight on the floor could be a big issue. Water is incredibly heavy and a tub 6.5 feet long would require a lot of gallons. It would require extra framing and maybe even a support post or wall to carry it.
I’ll let one of our math geniuses tell us how many gallons are in a 6.5 ft x 18 inches deep container.
Found this Vol calculator
I entered 6.5, 2.5, 1.5 and it says 182.08 gallons. times 8.35 = 1520 lbs. plus a 230 lb obese American male = 1750 lbs Thats a lot of weight concentrated in a small area of the floor. For a tub 30 inches wide and 6.5 ft long and only 18 inches of water.
http://www.dep.state.pa.us/dep/deputate/waterops/redesign/calculators/volcalchtm.htm
If you could lay down, drowning becomes more likely.
Yeah, but if you change the 6.5 in that equation to 4.5, how much weight are you really saving?
4.5 drops it to 126 gallons for the same 30 inch width and 18 inches water. 1052 lbs plus 230 lb man = 1282 lbs.
about 468 lbs difference. Actually I think most tubs are only 24 inches wide, so they use even less water. A luxurious tub would be 30 inches wide. 
I don’t mind if they make it narrower, so long as they make it longer.
Yet they have indoor jacuzzis that hold a hell of a lot more water than a bathtub, so I’m not sure the weight theory holds up.
Most tubs are 30" x 60". Six foot tubs are merely an upgrade, as are larger tubs of non standard shapes. We have put in some pretty large corner tubs that probably hold twice the water of a standard tub.
I really do not think the weight is that big an issue. Increasing the size of the tub also increases its distribution. Modern engineered joist floors are pretty strong, and it is not a big
deal to beef up flooring if required during construction. I lived in as couple old homes that had 6+ ft clawfoot tubs. Huge, deep tubs much heavier than a modern tub even empty.
I think it is mostly an issue of management of available square footage. Go look at some big houses with the giant ensuites and you will likely see some big tubs.
It’s cause standard tubs are designed to fit into a 60’ space and most people are taller than 60’.
No idea why that is though.
I see Kohler makes a soaker tub that is 72 inches long x 36 inches wide and 19 inches deep.
So if you got a six ft wide bathroom, it can handle this tub. I’d love having a soaker tub too. 
Here is the start of the Kohler online catalog for 6ft + tubs. There are eleven pages.
The answer is history.Think back to the first modern era bathtubs, the cast iron clawfoot jobs of the late 19th century. These things were luxury items as it was and damn heavy even empty. Moving and supporting ones bigger than 5 foot long would have been a major undertaking. Hell, trying moving a five foot cast iron claw foot tub without at least four big strong men. Plus by the time Koehler got manufacturing costs down enough to appeal to middle class 5 foot long was as big as would fit into a typical bathroom of the time.
It can’t be weight, cause, ya know, waterbeds.
Also, people were shorter a hundred years ago. I’ve seen museum exhibits of 19th century clothing and those people were typically much shorter. They’d almost fit in a 5ft tub.
Moving the damn thing around the house?
This is a good one. I remember a student, when I was working at college, falling asleep drunk in a bath one time. It’s not a stretch to think he might not be here today if the tub was long enough to slip all the way in.
Too soon for Whitney Houston jokes???
182 gallons? I see a lot of six foot tubs between 36 and 72 gallon capacity. At this site, you can sort them by capacity, depth, etc. The largest capacity is “over 80 gallons” but skimming through the first page (I know, not very thorough), I didn’t see any over 87, and those are not anything like standard tubs.
My tub measures about 10.5 inches from the bottom to the overflow, but I flipped the overflow upside-down to get a couple extra inches.