Yeah that's definitely a factor. There's part of it that's not related though – scaling out build capacity. Setting up a Jenkins build node is actually quite straightforward and reliable on the Jenkins side, similar to a BuildKite node for example, the issue is where is that node, how does it get provisioned, how is it managed, removed, etc.
For us, it was a bare metal machine where we had to email a sales rep to get a machine added, then spend ~2 hours setting up firewall stuff with semi-manual Ansible scripts. Add to that minimum contract terms and difficulty cleaning machines, and it was a pain to manage.
Conversely, if you've got a reliable autoscaling solution of some sort, and your build manager is capable of poking that as necessary to scale up and down (possible with Jenkins, but hard), then this could be really easy to do and BYO may be feasible.
Having a CI provider give us ~unlimited pay-as-you-go capacity that needed no management on our end and was always a clean environment, that was worth a lot to us in engineering time.
