Because resizing in a linear colorspace is more costly. JPEG can be resized without shifting colorspaces VERY cheaply, but requires loading into RAM if a change in colorspace (or gamma shift) is performed. The hit can be quite significant. On a phone or laptop it would hurt battery, on an online service (dynamic resizer service) it would impact latency.

Can also depend on the monitor? When I drag this page between monitors I see different effects.