Because people don't really understand game design. I'll try to explain how RS3 and OSRS differ.
RS3 has focused on high-end content for years now, as well as trying to reduce tediousness of skills in favour of higher impact PVE (see: EOC). The mods do not care about the balance of PVP and they would rather accelerate the rates of XP in lower skills so that they can introduce newer players to higher end content much faster.
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This is fine, and what most MMOs do because it's one of the best ways to keep sustainability in the game. People like to make the comparison of the Whips, where the Whip in RS3 costs only 100k. That's because the weapon is practically garbage in RS3, there are several tiers of weapons above it making it irrelevant whereas in OSRS, it's literally the best weapon in the game. Dscim is comparable to RS3's whip, and that also costs 100k.
Because of the greater focus on end-game content in RS3, which obviously holds much greater exp rates (as expected...), the RS3 mods decided to increase the max level of skills from 99 to 120, making it comparable times to max even with the greatly inflated XP rates.
Also, it should be noted that there's also a very large focus on removing monotony and tediousness in RS3. There are many updates that simply increase XP rates because they believe that their players, especially newer ones, aren't interested in playing low level content over and over when the game is so heavily focused on the end-game.
RS3 has the inflated XP rates on purpose because their game isn't a grind. It's an end-game focused MMO. OSRS has more "balanced" XP rates because the focus of this game is the grind, with fewer options in the end-game of what to do because the major gameplay focus is not the end-game.