We see ALL these npc with pretty cool looking clothes but yet we have ZERO access to them, why?
Because Rockstar wants us to look like sh*t and not give more variety that's already in game not available to us. If we find a way to actually do this it gets patched/hot fixed within hours or by the next update.
I get your complaints but it's really not as simple as you say, just because an NPC is wearing something does not mean the player character can wear but Rockstar's just restricting it, NPC clothing is specifically for the specific NPC model. If you tried equipping any NPC clothes on a player it would look ridiculous, e.g. the torso/legs/whatever would be entirely disproportionate to the rest of the body.