system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do almost every modern houses I visit have a crawl attic, as in an insulation floor, as opposed to a full attic where you can store stuff and walk around?

CONTEXT: I live in Alberta, so building codes might be different for me.
gpt
Most of the ones that had an attic were not well ventilated or insulated.  They had a floor over the ceiling joists for the floor below. If you pulled those up they had nothing in the space in the way of insulation. The house was constructed with a mostly load bearing floor/ceiling (the ceiling between the attic and the room below was probably NOT 2x4s) and the roof was built one-off with a ridge line and a lot of birdsmouth cutting to make the roof it self. If you go into one of these homes and look, the roof is going to be at least 2X6 for the pieces that go from the ridge line to the sides of the house. There will be fewer supports in the center of the spans as well.

New homes are built using pre-built truss for the roof. It is smaller dimensional lumber that is pre-made into triangles. The roof and the ceiling are actually one constructed piece vice two stick built pieces. They are not designed to have much load between the top (the roof) and the bottom (ceiling). They also have a lot more ties that go from the roof to the ceiling to tie the structure together. These keep the area between the roof and the ceiling cluttered and hard to have a floor. 

Insulation. The old homes probably had very little. Heat rises so having insulation in the attic is big deal for heating. Attics get solar heating in the summer and will generate a lot of heat, if not insulated it makes the house hotter. So to keep it warm in winter and cool in summer you insulate. Truss ceilings are usually only 2x4 for the ceiling joists. R-30 is recommended in attics now. That is between 8 and 11 inches of insulation thickness. So the insulation should be way thicker than the joists. So to put in a floor you need to buildup the floor surface which adds cost and weight or you need less insulation which will increase your heating/cooling costs.

Also with insulation most houses are now built with vapor barriers for insulated to not insulted space. They want the attic to ventilate to get the hot air out in the summer and to prevent moisture build up in the winter. (The attic is NOT inside the vapor barrier.)

This means the things you put in it would be exposed to temps that hit 130-140 easily in the summer and as cold as it gets in the winter. It may be exposed to higher levels of humidity than what exists outside. It is bad for whatever you are storing, and is not encouraged. (Basically an attic today is only really keeping the rain off.)