Not only, there is IMHO also a "technological supremacy" bias.

Quite obviously most developers will have:

1) VERY powerful hardware

2) VERY fast (and unlimited) internet connection

It's not like (they should do it as part of quality assurance or similar) they take a car, drive in some remote countryside, possibly in the middle of nowhere, stay there a couple days and try accessing their website (or running their app) on the lowest/cheaper entry level hardware on a metered connection.

It's easy when you have a T1 or faster connection on a recent top-hardware to forget how a lot of other people have slower devices, with less memory and limited bandwith and metered connection.