It becomes a technical challenge, involving latency, compression and robustness, to shift the relationship from home PC and handset to always-on remote server and handset.
Second, remember that when they decide to shut down a server for an always-on-DRM game, even if it's really just a single player game, it is legally unplayable.
WestClinTech launched a product that could do everything Excel can, sometimes more accurately, always faster, but placed it on a server running SQL instead of on a desktop running Microsoft Office.