Because for phones they kinda are custom. The smartphone hardware landscape is an absolute clusterfuck of proprietary blobs and closed source drivers and all sorts of shit that makes it so you need a lot of work to customize the base os to work on any particular device. ROMs have rather short lists of compatible phones, and each one of those had to have a build specifically developed for them. You can’t take, say, grapheneos and slap it on any phone you like.