Because they have no choice. At least not before next week, when the 1922 Committee has a chance to change the party rules.
Big fat hairy bollocks to that there's always a choice.
And anyone who believes otherwise is - frankly a bit dim.
Remember when Boris was potentially at deaths door in 2020 ? Mysteriously the choice was to put Dominic Raab (who is deputy PM, let's not forget) in charge.
In fact why the fuck bother with a deputy PM if they can't deputise ?
No, BJ clinging on is the final two fingers of the Tory party to those people stupid enough to have ever imagined that shower of crooks, criminals and perverts could ever govern responsibly.