But the point is they were never used for direct fire like a Sherman
they did
M7 Priests where sometimes used behind a wall of shermans. Even with the sherman 105. This was because the M7 Priest could achieve a faster ROF. And it had good protection to face minor threats
The M12 was famously used at point blank range in aachen.
The germans used Wespe's and Grilles alot at direct fire support, but dint had the means to protect them, and many where lost this way.
The russians thought about this, and simply made the SU122, SU152, ISU122 and ISU152. These had the roles of Self-propelled gun, Indirect artillery support and tank destroyer