Probably because bigger badder VPNs potentially let you side step tricky political and organizational problems. Bigger badder VPNs might not be the lowest cost

Probably because bigger badder VPNs potentially let you side step tricky political and organizational problems. Bigger badder VPNs might not be the lowest cost solution, or most efficient solution, but it's a relatively easy sell - you're just spending more money on the same type of resource as before because your using more of the resource. Anyone can understand that situation, no one is going to pushback. Switching to SaaS or cloud hosted providers, and especially if it means changing your security model means that you have to... do all those relatively hard and higher risk things to do.