It's just so much easier to stay and keep paying Microsoft than switching. But why hasn't any startup found a way yet to disrupt it by giving the users something they actually love more, so they can start lobbying businesses to switch to it, I wonder.
Excel is a behemoth of functionality which is constantly growing and improving. It's not as simple as building "just another spreadsheet"; and attempts are made, dozens of them. But it seems they all are either not really understand the strength of Excel, or simply admit defeat from the beginning and try to find a niche where they can compete, by looking at specific usecases and target groups. Kinda like all those companies who build hyper-specialized cooking-tools, but still can't beat the versatility of a good knife, so they mainly sell to normal people, not the real experts.
Because the power of Excel comes from the huge amount of features it has slowly accumulated over the decades of its lifetime. You can pretty easily make an Excel 97 replacement today, probably. But people won't be able to use that in these business contexts - they each need some of those obscure 1% features that got added.