It doesn't necessarily have to be about legally enforceable liability. Companies have insurance for that.
Even just from a data management standpoint, they have a business template that is much better than what could be implemented internally.
But after reading "Bullshit Jobs" [0] I think it's more than that - It's more about culpability and plausible deniability of decision makers in companies. Many companies have a CYA (cover your ass) culture. The higher up you go, the worse it gets.
When faced with a challenge that needs to be solved by tech, people in these companies have two choices: 1. Build it out internally, 2. Use Salesforce.
Salesforce is an incredibly safe choice for these type of decision makers. Because if shit hits the fan on an internally led project approved by them, they are associated with that failure. It effects their career trajectory. If something goes wrong with Salesforce - overbudget, data leak, etc. It doesn't matter, someone besides them is first in line to take the blame.
Ever hear the phrase "Nobody ever got fired for choosing IBM"? [1] Salesforce has grown to the point where they are recognized as a standard across industries by non-tech people. I've heard people from small local non-profits all the way up to publicly traded fortune 500 mention Salesforce. Name recognition on this level is quite literally worth $billions in the B2B world.
[0] https://www.goodreads.com/book/show/34466958-bullshit-jobs
[1] https://www.quora.com/What-does-the-phrase-Nobody-ever-got-f...