Because that’s a definite action.
If they wanted twitter destabilised, “giving” it to someone that is going to mismanage it (either deliberately mismanage it, or because they are an egotistical eccentric rich person who aspires to be a tech guru entrepreneur genius) is a much more subtle way to achieve it.

Adding:
Worst case, they have leverage over the company to remove/censor content. Even if that was their initial goal, having twitter collapse is still in their interest