Simple answer : To discourage win trading and to show difficulty curve.

Long answer: To prevent people from getting everything they want early with no consequence. If you could freely do all fights in any order without fear of losing chests people would jump to the last fight which holds the weapons/chest pieces.

Before you counter with just randomise where they are then you have people wanting the fight with their stuff 1st and if it is on an easier one like E5s compared to E8s it would cause nightmare fight balance issues as everything would need to be tuned to the higher end to prevent a lot more people having weapons week 1, just compare E5s clear numbers vs E8s clear numbers.

The devs have a clear you are expected to be able to handle E5s-E7s and have some gear from them to be clearing E8s. Pugs would jump to E8s 1st if they were not going to lose out on chests, they would be very unprepared for such a fight heck with the current system getting a pug clear of E8s is a dang nightmare (I've been trying for nearly 2 weeks with pugs and am now trying out a static, many a time could've killed but someone always dies costing the clear then group disbanded due to fatigue or frustration), having large groups of pugs with even less gear than now, no thanks the system can remain as is and I do not want savage tuned further down than it has been already if you are not prepared for what it asks of you, you don't deserve the gear, nor do you need it.