That is because most commercially made MULTI ROTOR aircraft are total garbage. Even the best of them always come up short cutting corners on some of the parts. But the main reason is a lot of the commercially made things are proprietary and you cant make any changes or adapt better gear to "upgrade" their inherent weaknesses.
This is also how they will establish that they can and do produce "compliant" aircraft to cater to the new rules no matter how unsafe they are with added unnecessary tracking and lockouts all increasing weight on on aircraft with already shaky components.
@KN4IUU gave you the perfect link to source parts for a build. Not only do they have respectable quality parts but they are also the only entity actually taking legal action against the F.A.A. destroying the hobby. Anything bought on their site helps them fight that legal battle on everyone's behalf. They are not in the game with smiles and compromise like has happened in the past while they take more away with each stage of the implementation of rules.
The multi rotors sold in chain stores are barely toy grade quality and should be avoided at ALL costs. Not only are they cheap, they are unsafe and most likely the first crash will bet the last on every one of them as they are made to be disposable and not repairable like hobby grade aircraft.
Just a heads up.. Multirotor aircraft are NOTHING like fixed wing other then they both use batteries and are remotely controlled.