And it's not as simple as you think, scale up has its own set of the problem. Power creep and trivialize gear progression are the two of many, and a low level player will suddenly find themself become significant more powerful when pair up with a high level player,
and that's not necessary a good thing. Forgot the names but there have been a few games that use dynamic scaling and most players hate it, 'cause it eliminate the sense of actual progression.
The reason a high level player think that kind of system is "fine" because it benefit them, but that benefit come as detriment to people who new to the games. Between some "minor" inconvenience to the max player (and face it, most even don't run into that often) or destroying the lvling process of new players, it's very clear what should take a much higher priority to preserve.