The point of limiting the number of pieces of equipment are to give you options to use at least three things, but NOT the option of enhancing your tank too much.
We each have things we can choose for different reasons based on our playing styles etc. but there are limits placed in order to make you choose wisely depending on the tank & play style.
The game is about thinking; its strategy as much in the game as is what we choose to put on each tank. That's part of the point; strategy. What good would it do if we could choose all of them?
Some tanks require a Camo Net while, others require a gun rammer, but therein lies the choice. You have to sacrifice somewhere, making it a CHALLENGE, which is part of the game.
Each tank & player is different, even when two players choose the same tank, often they will choose different equipment choices.
For some, the gun lay drive is more important than the gun rammer.
I'm curious to know where you got your stats on what is MOSTLY USED BY PLAYERS. I don't believe what you said is even close to correct (other than the part of the Trousers lol)
One player will choose Spall liner on a tank that tends to lose crew members a lot, or sees more damage from arties than someone else playing the same tank.
Spall liner protects the crew from injuries from explosions & being rammed but if you do not play your tank as aggressively as someone else does, you may not benefit from using the spall liner at all.
In some cases, spall liners are best suited for heavy tanks higher up the tier chain but for myself, I have found them to help keep my crew from having to be HEALED so often so in some cases I'll choose that over say, a gun rammer for e.g. I depends on the tank. There is NO DEFAULT 3 pieces of Equipment to use on every tank & if you are always choosing those three nearly every time for every tank, you clearly do not understand how they can be used or for the very reasons they are available.
I find my tanks often differ from that of other tanks I'm tooning with when it comes to choices of equipment used when in the exact same tank. Each tank has equipment on it which compliments that specific tank.
Don't make assumptions about how equipment is used by others as many players (like myself) often choose differently than what I would call the status quo & for very good reason.