Excellent question!

First: editing _attributes_ of geometries is totally possible, editing the _shape_ of geometries is not possible with MapComplete.

If one allows to edit shapes (aka: geometries), this means that one can draw or move lines around - for example: add/move a cyclepath. A contributor might thus move a line into another geometry (such as a building). But as MapComplete has focused maps, they might not be shown to this contributor!

Allowing to edit geomtry shapes this implies I should show all _other_ lines and polygons (e.g. forests, streams, buildings, ...) to avoid incorrect intersection.

The point of MapComplete is to keep it simple and focussed on a single topic. Showing all geometries is counter to that. On top of that, we already have an excellent tool which handles geometry drawing: the iD editor you'll find on openstreetmap.org/edit .

One could also argue that creating/moving points (which can be done with MapComplete) could also cause data errors; but the 'add new point'-wizard has some steps to invite to add with high accuraccy. And even if someone adds a point in a wrong location (e.g. in a building instead of outside of it), the impact of a misplaced point is generally way less then that of a misplaced line or area.