Square peg Round Hole
To me game and system are separate parts of the RPG experience. For most of my life I have played epic fantasy games, most of which used D&D. That works because D&D is a square peg, and a fantasy epic is a square hole. When someone wants to run something specific, like a dark horror fantasy epic, there are enough add on pieces to D&D to fill those new angles on the hole. This is one of the (many) reasons why 4th Ed didn't fare as well as Pathfinder, or 3.x among most gamers. When WotC came out with 4th Ed, we went from a pretty standard sized square peg to A MONSTROUSLY HUGE BLOCK OF GRANITE THAT MUST BE CONSTRUCTED USING MANY SMALLER PIECES OF GRANITE! And if you bought all the pieces to go with that giant granite block, you could build a giant square hole that the block would fit into nicely. But for people who already had a hole... it just didn't work. 4th Ed was too difficult to mold into different campaigns, where as 3.x and Pathfinder could be hammered into just about any hole by shaving off the parts that weren't working. Also, there were tons of add-ons to fill in empty spaces.
Sometimes you have a star shaped peg: I want to run a mech themed furry game with magicians and ninjas fighting fairy princesses from Mars across an intergalactic battlefield littered with pop culture references and deeply emotional character interactions with a cast of NPCs made up of from Steven Universe and My Little Pony: Friendship is Magic where all the gods are cats from the internet and people use puns as currency. When there is no system built to fill those corners, you go with a very small peg - a system light enough on rules to allow anything and everything. Systems like Unisystem and Savage Worlds thrive in this environment because there are enough rules that a good GM can corral players within the scope of the game, but allow enough creativity to keep the game fun.
And sometimes to use a particular system you have to compromise on the game. For example, it's a pain in the ass to run a low magic game in 5e, because literally every class has some access to magic. To shave off the magic components the game would become unbalanced and no longer be much fun for players. So to accommodate that system a GM has to allow that most PCs are going to have magic in that low magic game. Which can totally work: PCs are PCs for a reason. Usually a PC is an exception to the rule.
And then there's always the option of making your own system.... While this might be the most challenging of paths to take, the results can be amazing. I ended up making my own system to support the game that I want to run, and so far it's worked out pretty well. My combats go the way I want them too. My PCs have the abilities I want them to. I can make monsters the way I want to. It takes time and dedication, but eventually you end up with exactly (or pretty close to) what you want for your game.
And last, but not least, there is the internet. There is nothing new under the sun. You think you have a super original idea for a system using cards for combat? Clockwork Dominion. A system that only uses d6? Shadowrun. A system where you use a different polyhedral die based on proficiency in that still? Savage Worlds. A system only using d10, but you try to achieve a base difficulty on multiple dice and count number of successes? World of Darkness. A system with just story! QAGS. I could go on. Everything exists in one form or another, and if you spend enough time on the internet you can find anything.