Very simple reason: If you have a "studio" which is made up mainly of permanently contracted employees you have a constant drain on your finances. Taking 6 months to get a new project signed/greenlit and you can find the coffers drained, putting yourself in the position where you are reliant on advances to make the initial milestone on your next thing.
The bigger the headcount, the higher the burn-rate. So naturally studios which have accumulated a large staff from previous high-profile projects are the most at risk of this.
Once you sell-out to a big publisher, you are entirely at the mercy of your owners because any project you may come up with can no longer be shopped around. So, if your last project underperformed and they don't see any merit in what ideas you come up with next (or have anything of their own they want you to do), its goodnight sweet prince.
The reality is that studios are rarely bought for their staff or expertise, its typically because they have an IP that is seen to offer sufficient value up-front to justify the acquisition. This is why a high-profile franchise entry failing can be such a terminal event. The corporate perspective is not just that you may have lost money on that single entry, but you also have devalued the franchise making your existence moot.