Maybe because it's for an Airsoft gun. The site "airsplat.com" is an airsoft site. 300 rounds of airsoft pellets, sure, but that definitely isn't a 300 round MAGAZINE.
I mean they do have 100round beta mags, but those are in drums, have been known for feeding issues more often than not, are pretty expensive and obviously heavy and cumbersome. Some people may have them, but the majority of them don't
Standard size of 20-30rds, you know, what is actually made OEM and has been for decades are just fine and what we hate being labeled high capacity CLIPS.