Were mask ROMs with stock contents ever sold? or why not?
Character generator ROMs with 5x7 ASCII codes were certainly sold. 

One example was the Signetics 2513, a rather nasty and sluggish P-MOS chip requiring three supply rails. 

OTP and UV-erasable EPROMs came along not so long after- so the cost advantages of mask ROMs were not as significant as compared to mask charges (mask ROMs are not really fully custom silicon, only a metal layer was customized). Typical costs were low thousands of dollars and a MOQ in the thousands from US suppliers.

There were also small TTL OTP ROMs made with fuse technology. 

And there were (and are) 4 and 8-bit micros pre-programmed for specific tasks, perhaps with an application-specific peripheral or two.