Why?
Well for one they very much narratively fit in there, being a product of steel, iron and stone.However the main point for me is that foundries provide an almost disappointing lack of support for Purple/Production Science. Notoriously the most steel and hungry science there is.
The EM plant inherently boosts all sciences but the Foundry can "craft" almost nothing as it mostly processes molten metals. Making belts is great and all but the EM plant instantly upgrades your base by boosting chip production. For the metal boost you have to redo your entire smelting setup while importing calcite since getting calcite from space is locked behind Agricultural Science for some reason.
Also, elevated rails are insanely hungry for refined concrete and the 50% productivity bonus would make using them more enticing.
Tl;dr
Foundries should be able to craft more than just belts and rails are an easy choice without being overpowered.Things like Gears, Sticks and Pipes (again, actual crafting, not using molten iron) would be neat as well. Compared to the EM plant giving you 50% on all circuits I doubt it would be too broken.
By the way, I mean actual crafting (like the belts) not a dedicated recipe using molten iron.