Said skills and tools are both less than common and scale worse. Scale also makes things cheaper counterintuitively to hand crafting. If you have say a drill press that can easily drill every hole at once it makes for better logistics to just drill every hole and stock them than keeping a ton of variants or worse drilling on demand.

I can certainly understand that mental blindspot however given I can't help but think of enterprise software as drastically overpriced for stuff I could slap together myself.