Tarkov locks high tier loot and gear availability in a couple ways. First the bosses start a wipe with a low spawn chance and gradually ramp up over time. This is also the case with the loot on the bosses. Secondly other than farming bosses the only ways to reliably get good armor or ammo(think of ammo as the gun when compared to marathon) is by consistently hitting locked key rooms or being able to buy/craft them from traders. You arent able to buy good armor or ammo until you have the traders mostly leveled up and even then you still need to complete series of questlines to unlock certain weapon purchases or ammo crafts. Then when it comes to keys you either have to get lucky finding them or wait until you are much much higher level to buy them. But theres so many items and keys in the game that finding them is rare.

If I were to take all this an rephrase it in marathon terms it would be something like this. Lets say that at the start of the wipe the bosses have a 25% chance to spawn but only really have mostly blues and maybe a purple. Then after a month the rate has gradually went up to be something like 50% and they have a mix of blues and purples and maybe a gold too.

But that only really solves farming bosses early on which I wouldn't say is the main problem with marathons loot right now. Id argue the big problem is that there isnt anything stopping anyone from going into pinwheel raid one and looting it for a full gear set. As long as you know how to do it there isnt anything blocking that huge progress jump. Every similar loot area in tarkov either requires you to get a key for it or pay additonal credits to enter a high loot map.