I would posit another large factor is "owning" the software comes with the long tail of edge cases, bugs, support, on-call, regulations, etc... that an established SaaS has learned and iterated on for many years with many customers.
For the vast majority of companies they would (and should) rather let the SaaS figure that out and focus on their actual company
AI can allegedly do all of those. Except perhaps it cannot be trusted with regulations, since that one has consequences for doing poorly.
It seems people are forgetting that companies should develop their differentiators and pay for the rest.