One reason would be revenue recognition. Your project is expected to deliver $X top line revenue on such and such a date. You have to make that date if you don’t want to get sued for securities fraud.
Another reason: Software is never done. You have to ship it at some point.
The last numbers I saw say the home market is 9% of PC sales, with home builders being a small fraction of that.
Anecdotal - I know two adults and two kids that have assembled their own PCs. Everyone else - including me - buys off the shelf or CTO.