I think part of it is the problem space to be honest. SAP basically gets used to glue all the fiddly bits of organisations together and I don't know that there is an approach to that set of problems that doesn't get tangled up in corner cases.
With a lot of the nice looking web based apps we're used to looking at, the UX and the target domain evolve together and if there isn't an elegant way of doing a particular operation then there's going to be pressure to drop that operation entirely. SAP deployments always have to do everything in the organisation so complicated corner cases either get dropped (bad) or end up with suboptimal interfaces (also not great!).
With a lot of the nice looking web based apps we're used to looking at, the UX and the target domain evolve together and if there isn't an elegant way of doing a particular operation then there's going to be pressure to drop that operation entirely. SAP deployments always have to do everything in the organisation so complicated corner cases either get dropped (bad) or end up with suboptimal interfaces (also not great!).