Short answer: Because so-called "experts" have pronounced that there are various purported benefits to the FLGR, manufacturers offer them because many buyers have been convinced that they "need" them.
Want one? I have a drawer full -- any 1911 that comes through my door with a FLGR gets an immediate conversion to a standard 1911 recoil system as the first item of business.
a while ago I got a govt Springfield that came with a 2 piece FLGR. I hated it, the disassembly process. I had a 1 piece FLGR on a old colt that I could remove without any tools assembly but still a pain in the finger, that I added to that pistol because I "thought" it was appropriate. Hating the 2 pc I got to thinking and reading, I was cautious of the idea of converting both to GI style... if Springfield added it on their new gun I thought there must be a reason... hence my question here why are they doing it?
I searched and read and googled and asked found nothing. I also laughed off that Brownell's video at 0:12, the GI system fully captivates the spring on the I.D. of the plug and the O.D. of the GI rod... any coil spring would never work at all in any application if it kinked anywhere in the process. Also, yes while you can remove the slide as a unit with a FLGR you better not bump it when you set it down the FLGR isn't holding onto much against the barrel lug.
The general consensus among experienced 1911 shooters was it was worthless. So I bought 2 GI plugs and short rods and brought the old Colt and the new SA back to original condition and love it. No tools, no pain in the finger, no performance loss, no looking back.