Because hearing or reading that’s the reason for doing less dps than a low-geared healer or why they didn’t move out of the bad stuff time and again is beyond old. As is someone “dc’d” and the reason is “my comp died”. And that’s not even accounting for Arena. Their toaster is dragging down the rest of the group, but they don’t care.
If they stuck to solo content where performance issues affect them and them alone, then no one would care. But when other people are involved, those things are likely to be a hindrance to those other people. At best making it more difficult to get the job done, but more often than not causing so many wipes (unless you’re paying for a carry, but that’s a different story).