MOS couldn't keep up pace with the rest of the world. Commodore's foundry was great until around 1985, after that it kept lagging behind and in the case of the A4000 and A1200, they started letting others companies like NCR, HP and VLSI fab the Amiga chips because it was either too demanding or too expensive to do at MOS.
But doing so is obviously an option – ARM has never had a foundry of its own yet it is very competitive and profitable. What really can cost money is to actually develop the chips you want to manufacture. Commodore didn't have the money to do so.