This may turn out to be a stupid question, but I have nothing to
prove...

I don't understand why it isn't possible to make roofs cheaply, using
board materials instead of tiles or glass fibre. It's possible to make
boats waterproof, with appropriate sealing materials and pain, so why
not roofs, which have to deal with a lot less water?

It would be convenient and cheap to put large plywood boards on a roof,
make sure that there's some kind of overlap or flexible seal where their
edges meet, and finish them with thoroughly waterproof paint.

I'm assuming that I'm not a genius who has thought of something that has
never occurred to anyone else, and there is a reason why this isn't done
- what is it?

Daniele