Historical consistency and preference.
1. Stacking only arrived in Factorio 2.0 and even then loaders didn't have the ability to stack output.
2. Some consider loaders in general OP, others consider stacking to even more so.
- Default for non-Py games is that all loaders do not stack, and there is a separate stack loader like inserters
- The Py community's opinions are stronger in this area, so the default for games with Py is no stacking enabled. Py Stack inserters are not 1x1 entities either.