There are many reasons for having rule sets in TTRPGs. A critical one is for communication and standardization. If you meet a group of people and they say "We're playing a game of E&E 2nd Edition", and you are familiar with the rules of E&E 2nd edition, you should be able to sit down and play at the table with minimal effort. There may be some difficulties is working out the details; every table will have at least a couple house rules that need to be properly communicated for players, and there will always be social issues to consider. But for the most part, once a you have identified the rules set being used (including edition) the hope is that you have also communicated enough information to start sharing a game.
Power creep makes that difficult. When multiple rules supplements are added that continually increment the power levels of the players, it means the game is no longer standardized. At some point, you can no longer communicate the rules of the game just by saying the name and edition. In D&D 3.5e for example, playing a core only game is very different from playing a game with the first generation of splat books (i.e. the class books), which was even more different from playing a game at the end of 3.5e with the Book of 9 Swords, Magic Item Compendium, and PHBII. You cannot just say "We're playing D&D 3.5e", you have to say "We're playing D&D 3.5e with this list of allowed supplements..." or a player will not know how to start building a character.
Power creep is, essentially, bad for communication. The more power creep a game has over time, the more specific players have to be with others to ensure they're still playing the same game. When power creep happens fast, the power changes to be communicated quickly and accurately across the entire player base, or the player base will start to fragment. When power creep gets large, its is critical to document that power creep for new players, or they will learn only the base power level of the game and be more confused when they try to enter the gaming community (that is, power creep can be a form gatekeeping).