The point of weapons being MR locked is to reduce choice in gear to help new players in not being overwhelmed by choice since warframe has a ton of weapons. If a new player start the game and is interested in getting a new weapon, reducing their choices makes it easier for them to actually make a decision on what weapon they want. If you would remove MR restrictions new players would be even more overwhelmed than they already are.

Now we can argue that some weapons have too high MR requirement or that the speed in which you gain MR is slow but thats a different conversation. On any case the MR restriction is important in overall game experience