I don’t think it’s inherently easier to hack than any other system, really. People are just very willing to hack it, because they’re very comfortable with the base rules, and because D&D has had a DIY culture since its inception.
I don’t know about that. I’d say GURPS is harder to hack than any edition of D&D (you can add skills and advantages or disadvantages easily, but actually changing how the game works is harder), and 3.5 is harder than 4e, and both are harder than 5e.
3.5 especially hard because it is meant to be run in a way where balance matters to player enjoyment, but is incredibly poorly balanced, on top of being complex, nitpickey, and hyper specified.
4e is hard because most big changes would require rewriting immense amounts of player options, or reducing the game dramatically to avoid that work. We did hack it a bit, but not nearly as deeply as with 5e, in spite of the fact that I and at least one other dm in my group understood that system
completely, and definitely had things we wanted to change.
5e is balanced in a way where some pretty hefty changes can be made without any need to rebalance the game, the systems involved are very simple, and the same math balances everything. In play, the rogue and Wizard are roughly balanced against eachother. That means we can take per day stuff and make it at-will and actually make it work.
The only system I’d say is easier to hack, because it’s a system you use to make a bespoke game, is pbta, but IMO it isn’t very easy to make an individual pbta game with the breadth of play experience options that D&D has, so the only reason I’d use it instead of D&D would be if I enjoyed it basic mechanics more than D&D’s.
Hell, I’m building a system from scratch, that is built to do a less “superhero” but still quite pulp fantasy adventure game, set in a version of the modern world where magic and folk lore creatures are secretly real, and there are 8 other Worlds besides the mortal world. It’s all skills and traits, with quick build archetypes and ancestries, and attributes are resource pools rather than directly altering your dice rolls, which are a dice pool using skill rank dice added to an “action die”, using a success ladder.
Gameplay is focused on investigation, use of contacts, favors, assets, getting the help of communities, and using a broad range of skills and specialties to solve problems, in a world where the train conductor who is secretly a forest troll can save the human townsfolk from a group of knights of a “holy order”, and succeed by gathering allies, calling in favors, leveraging contacts, bartering with assets, and investigating, researching, and/or training, with a focus on the threat.
So, it’s not like I’m over here only playing D&D and games very like it.