Well, they probably have some kind of tile "lint", need to precompute a bunch of routing data (depending on what their actual algorithm is), sync all their rendering/prerendering pipelines at a common revision, propagate outputs to cache servers, etc. So it can't easily be done at every change, unless they have a design where small incremental deltas are explicitly supported by all their services. It is a time- and resource-intensive process. Still, three months sounds like a lot.
The concern seems to be moderation (but they haven't made a clear statement about it either). The current pace of updates is slower than what they were doing a year ago, so it probably isn't a technical thing.
Their routing engine updates more frequently than the rendering data.
OSM sub-10 minute updates really depends on zoom level and data. With low zooms the update can take weeks even there, as I recently learned in a hard way. It is more important to have consistent and laundered map data than someone's last minute changes.