And they have at time done just so, especially Team Pixelboy and Opcode Games. Both aforemntioned Homebrewers have also struck deals with AtariAge to manufacture and distribute a large number of titles, but things take time seeing as all or most all do this as a Hobby and have full time jobs, families and life to live.
When they place their orders for cartridge parts for their next new game, they can order the number of extras to fulfill existing requests for previously soldout cartridges. If they want to take a small risk (seperate from that of the new game) they can order a few more to fulfill future orders.
While almost all are never released as a rom file at release, most are eventually shared freely to be used via emulation or a MultiCart.
Aside from all the above, this whole subject eventually boils down to what and how the Homebrewers want to go about doing things and I'll always respect therr decisions even if I don't always agree with them.