Probably because it’s relatively very expensive, and very tedious.
You ever step on a Lego? Do you want a whole floor made out of 'em?
Did they have the standard connectors, or were the attachment points oversized on these oversized Lego bricks?
Heh, you got there first. I was going to suggest paving the floors with the smooth topped bricks as a safety measure 
Everything was scaled up. It was a big brick about 2 inches by 4 inches, with eight big studs (each with the Lego logo embossed on it, just like a regular Lego piece) on the top.
I remember red, blue, yellow, and green. I also had some square ones with four studs.
Got pics Chronos/E-Sabbath? This is reminding me of the I saw the Biggs on Tatooine scene in Star Wars when I was a kid! thread. 
I have no memory of “Giant LEGOS” in any of the LEGO catalogs I drooled over in the 70s and 80s. Earlier or later? Test market maybe?
Stupid parents never bought me the Galaxy Explorer.
I can ask my mom the next time I talk to her, but I’m not certain we even still have them, and she’s sufficiently technophobic that it might be hard to get pictures from her. I got the impression that they were pretty old, though: I was playing with them in the early 80s, but most of our toys growing up were used multiple times over, so they might have been out of the catalogs by then.
Mine are from the late 70s. Children’s blocks, I remember one was a duck. Like the really old ‘duck on wheels’ toy, except you could make it out of lego.
… Maybe I made the duck myself.
http://guide.lugnet.com/set/?q=2411_1&v=z
Turns out it is Duplo. From '74.
They have house brick sized Lego at the Melbourne Museum (see pic about 80% of the way down thisblog). They’re made of rubber, not standard Lego plastic. You can build a preschooler-sized playhouse quite quickly and easily, but I wouldn’t want to try a real house - they come apart far more easily than ordinary Lego or Duplo.
Giant Lego blocks made of (hard) rubber. And a view of the underside of the same bricks.
A desk made of standard sized Lego. (With what appears to be a Lego grandfather clock in the background, but I can’t be certain.)
The ones I’m remembering definitely weren’t made out of rubber, and I think they were a bit smaller than the ones in that picture (though it’s hard to tell exactly what scale the girl playing with them is at). They stayed together pretty well-- I routinely built “rocket ships” with the bricks on end, that stood up to 3 or 4 feet tall.
Yup, that’s a LEGO clock, all right.
Are you allowed to use some sort of glue?
Lay the flooring Legos on their sides, smooth faces up, pegs laterally.
James May stole our idea! 
Bastard!
Here’s some pictures of James May’s LEGO house, currently under construction. Note the wood framing used to support the walls; it doesn’t quite meet the OP’s criteria in that regard.
Also, while I appreciate the old-school mentality that led him to construct it in the five classic colours, I do have to say that it’s ugly as sin.
(Re the link above) Since when were they called “Legos”? That’s just silly. Lego is and always has been a mass noun. You don’t have Legos any more than you have a bowl of rices or popcorns.
“Legos” is the typical usage in the US, at least. I’ve tried to get out of the habit of saying it since learning better, but old habits die hard.
Just wanted to reopen this to share this video of the finished house. Wikipedia says it was demolished a few weeks later.