in 2012 the 'Rooney' shirt became the highest selling replica shirt in Premier League history.
He is in Nike ads around the world, Turkish Airlines ads, Singha ads throughout Asia, DHL ads for Man Utd. He is everywhere. As ugly as he is, he is a marketing dream for them.
Don't forget he wanted to leave Man Utd quite recently, until they paid up. The tried and tested Lampard/Terry tactic to panic the club into acting. They can't lose the face of their club. Only a few can afford him and his style limits where he can go - his aggressive play won't work in Spain where he'd be sent off every other week, only PSG or Monaco could afford him in France, and Bayern in Germany but he's not smart enough for a Guardiola team. So, Chelsea or Emirates Marketing Project. Tried that, didn't work, he's 28 and this will be his last contract.
He is unlikely to reach the peaks of a few seasons ago but he's still very good for them, he will decline and they will be left with a very expensive asset - as long as he sells shirts, do they care? Man Utd spend 400k a week in interest on their loans, as long as the return exceeds the expense they are fine, and having Rooney helps that. Just note the number of commercial deals they have signed in the last few years; car tyres, watches, snack food including a Pringles replica and some noodles in Asia, car partner. All sorts of really random items around the world.
Who can Man Utd find that is better for them at this time? It's opportunism from Rooney and his agent, and it's desperation from Man Utd to hold their big names.