1. They think that the civil service has become not just openly hostile but outright dangerous to any form of Republican government, and therefore that taking direct control of the civil service infrastructure at high speed is essential to avoid being kneecapped by rogue federal employees again. They think that this happened during Trump's first term, and that if they don't get this problem under control then America has effectively become a Democrat dictatorship that does whatever the left wants regardless of who wins elections. They have a good reasons to believe this is a real problem they need to solve and fast, see Sherk for some egregious examples [1] but there are many more you could cite.
2. A genuine belief that the government is very inefficient and in particular that a lot of the waste is basically just funding the Democrats via various 'laundered' routes like allied NGOs that pretend to be politically neutral charities but aren't. Doing something about that is a good way to get libertarians like Musk and his allies on board. Everyone is in favour of government efficiency in principle so letting the libertarian types go cut waste is an easy way to build that coalition even if the other parts don't care about fiscal efficiency much itself.
These two are interlocked. Poor performance and efficiency improvements are one of the legal justifications for laying off civil servants, so it's much easier to get the civil service under control if #resistance results in being one of the ones "optimized out" of a job. That's doubly true if the sort of NGOs that would hire them if they were fired are being defunded simultaneously.
[1] https://americafirstpolicy.com/assets/uploads/files/Tales_fr...
