Buy a fecking season ticket in the Stretford End or singing section then, and turn up week in and week out and sing your heart out for 90 minutes. You're either part of the problem or part of the solution.
People can make all the excuses they want, but supporters have agency in making a great atmosphere in the stadium. You buy a ticket, turn up and show your support. The atmosphere lacks because not enough people want to do this. That's it.
You don't want to travel to matches because it is too far or because it costs too much? Fine, but realise you contribute nothing.
You want to sit in parts of the ground with the premium views, or catering, or where you won't be obliged to stand, or won't feel awkward about not singing? Fine, you are entitled to do that, but you are part of the problem diluting the atmosphere.
You want to come to the Stretford End for the "experience", or to film the corner kicks on the chance of owning a video of you and the stand celebrating, or to take all the selfies you want with your half-and-half scarves? Fine, again you are entitled to do that, but again realise you are cannibalising the experience that you are seeking out.
You want to come to Old Trafford, stand for the entire match in the Stretford End, keep your phone in your pocket, sing every chant and song, attempt to start some of your own as well, and then go to work on Monday with a hoarse voice? Great, you are making a positive contribution to the atmosphere at Old Trafford. There's not enough of you.