It's impossible to generalize an answer, since everything depends on a specific and individual design and on a family's lifestyle and priorities. In many designs, the area over the first floor house may be devoted to second storey bedrooms and secondary uses. That leaves the garage for "bonus" space if the owners want to finish that space for habitation and use.

For designs that are strictly one storey usage, often the challenge is how to plan and locate the necessary stair for second level access. Gaining access in the future to areas above the first level living areas is usually a major challenge, unless planned in the very beginning.

Tract homes, more often than not, are built with roof trusses for the entire roof, making usable "bonus" space anywhere above the first floor impossible.

Everything depends on the early design studies and decisions.

Good answer from Virgil there. Many tight conditions call for them but I like to spread the word on the dangers of attached garages. Try to avoid a garage, if not build it detached, if not detached be aware of the indoor air quality concerns that come from attaching, especially the spaces above them.

I don't believe the reason is related to space planning but is in order to take advantage of the lower cost and erection speed of long span roof trusses. The span of the roof framing over the garage is usually much shorter and a stair doesn't have to be in the front of the house. Its a developer configuration that appears to have become a standard even for custom homes in many markets.

I see many one-story home designs with a high pitched roof that have either dormers or other vaulted ceilings used in the design to bring in more outside light. This would reduce or eliminate the possibilities of a second floor over the main living area.

We're building a one story with high pitched roofs and a bonus room over the garage. The high pitched roof is because we're going to have vaulted ceilings in the living room and master bedroom. The secondary rooms will have tray ceilings. Those ceiling remove any usable space for a second story. The bonus room is for the kids to hang out. They can get some what noisy without mom and dad complaining since they are not near us or above us.

It's often cheaper to use trusses over the main house -- and the higher roof makes the house look more impressive to some. I wouldn't want any 'bonus room' on a separate level unless there was at least a half-bath on the same level.