Because a lot of times, you're just paying more for a name.
Welcome back, trouble maker.
So you don't feel using noveske, bcm, larue parts are better than say PSA or spikes? So are all the people buying bigger name brands wasting money?
Well, I've built a number of lower receivers, including two more this weekend. They have used a variety of lowers from several different manufacturers - Rock River, Mag Tagtical, Spikes, Rise and Anderson. Uppers I have used from Rock River, Anderson and Aero Precision, as well as some unbranded "keyhole" and "broken A" uppers. Personally, I haven't found problems with fit or finish on any of them. Not a single one of my rifles has ever had any "slop".
The only complaint I have had at all that was anything other than cosmetic was the Mag Tactical lower. When the upper was released from the takedown pin, and rotated forward on the pivot pin, the bottom edge of the Troy Alpha rail would gouge the front end of the pivot pin housings on the lower. This wasn't bad execution, just a design that allowed those little nubs to stick out a little too far on the bottom. Even that was cosmetic... doesn't affect functionality or anything at all. What are those things called, anyways? lol
Are you paying only for a name? Well... if it all fits together and works just fine and locks up tight... then the only thing left to pay for, that I can think of, is name, aesthetics and possibly additional features. I don't particularly have a problem paying for any of these, if that is what you want. If your Mega Arms lower has the billet look, the textured and flared magwell you like and a pretty rollmark and you're willing to pay for those things - then more power to you. Some things I will pay more for... some I won't. But don't try to tell me it shoots better than the RRA or Spikes ones I have hundreds or thousands of rounds through just because of the lower receiver - barrels and such, we can talk about. But not the lower. The upper... mmm... I have yet to be convinced, certainly never had it proven.
To each his own.