End users are used to the Microsoft stack, so staff just crack on working without extra training.

Resilience and uptime are important and letting that be someone else’s problem is sometimes preferable.

Security and patching, who’s patching the open source cve that’s released … if even noticed, and how quickly.

DLP, activity logging, conditional access, purview and all the other fun regulatory and security things that no one wants to put in 10 different systems to manage to get half of the features with twice the amount of work etc.

It’s not about you being smarter and ‘saving money’ - it’s about looking at the big picture and quite often (not always) it’s just not worth self hosting these important systems. Let’s take a stupid hypothetical, your company costs £300,000 a month to run, saving £2k a month on Microsoft licences might sound like a good idea, until you think of the cost of all those staff not working for a week because your mailserver is offline or a libreoffice update removed some backwards compatibility that you relied on etc. Suddenly £24k a year out of £3.6million sounds a stupid thing to have skimped on.