Because usability is measured by the naive expectations of the least sophisticated users.

But haven’t you seen the least sophisticated users trying to type in the same sequence until it sticks?

Because most of the time you actually want it to be a date and those are stored in the spreadsheet as integers

You can add a single quote before any value to prevent Excel from autoformatting

Or just turn off autoformatting entirely

In this case, because it's much faster to enter data such as dates by entering the shorthand - e.g. 6/8 - than have to type 06/08/2020. The same reason it's very helpful to type "500" into a currency cell and have it convert to "£500.00", or the many other sorts of autotype.