We don't want provability or inference as the first approach to problems - slapping something together with a mix of mutable/functional code is Good Enough to start understanding the domain. But once the solution is complex, and it's necessary to improve iteration times and lower error rates, it tends to be too hard to jury-rig deeper logical properties into existing languages; it leads to mucking about with the build system and compilation process at a deep level, which adds a ton of extra complexity. The "now you have two problems" quote comes to mind.