It's less builder profit and more "how can we maximize the things people compare". Builders hate finicky stupid shit [1], and charge accordingly, but big empty rooms are easy. People ask "how much house can I buy" and get $400k, say, and choose the largest sq ft with the amenities they want, instead of smaller with more design/amenities.
And that's not even to say they're always entirely wrong; you should spend on the things that are expensive to change later (location, foundation, etc) perhaps; like a church that is built with plain windows because you can always add stained glass later.
[1] an example my friend hated as a roofer were the little decorative things on the corners of a roof (corner gables? no idea) - he charged something like $2k per because of the absolute huge amount of absolutely fiddly labor required to build and roof them, and hated every minute of it.
And that's not even to say they're always entirely wrong; you should spend on the things that are expensive to change later (location, foundation, etc) perhaps; like a church that is built with plain windows because you can always add stained glass later.
[1] an example my friend hated as a roofer were the little decorative things on the corners of a roof (corner gables? no idea) - he charged something like $2k per because of the absolute huge amount of absolutely fiddly labor required to build and roof them, and hated every minute of it.