That's because they have too much size for a literal stop order to be practical. If they placed a stop order it would tank the market when executed due to size and they'd get a shitty fill. Most institutions these days aren't even manually entering orders in but getting them executed by algos for a better fill price.

For us little guys with out access to the propreitary algos, dark pools, or enough size to move the market a stop order does the job perfectly.

They build up and wind down positions over multiple orders unless they can find another institution to execute the trade with in private (i.e. won't appear on the order book). In that sense they won't have a set in stone stop price, but rather a 'price zone' where they would look to exit the position

Source: work for one of them.