Because the Glazers believe that the club will be worth more in a few years if a few specific things happens. (Most likely primarily the Saudi involvement in football transforming European high-level football into a more profitable venture. Similar to PGA/LIV that happened in golf.)
But the club also depends on further investment happening now, so they need someone to come in now, even if there's a chance it'll all be worth more later.
I have intimate knowledge of another company (not football related) currently being sold in a similar situation.
The company is desperate for a cash injection, but current owners believe there is bigger value to the company based on certain things in motion. So the company is getting another investor essentially buying control of the company, but the final price to the current owners depend on if certain things pan out or not in the next year(s).