You can ask that same question regarding why other american club owners have NOT taken dividends from their respective clubs, for example Arsenal and Liverpool.
I get what you are saying, but there is a big practical and cultural difference when it comes to a professional football club in Europe, vs. a company that only exists to create a service and cash surplus for investment.
Companies are in their right to not pay out dividends. And quite frankly they very often choose not to, be it in times of financial hardship, general underperformance or a need for shareholder investment that makes paying dividends in the short term a very bad optics move.
While the Glazers are in their right, technically, to extract dividends from the clubs profits, the optics are god awful, and the damage to the club itself, their asset, is big.
Firstly, taking dividends from the clubs cashreserve directly impacts the clubs ability to negotiate the transfer market, invest in infrastructure without adding extra loans and generally anything else you can do with £11 million pounds in the short term.
Manchester United is strongly in the category of "generally underperforming". A good club owner would provide both financial leadership and provide a will to invest in the club. The Glazers have not invested a dollar into Manchester United.
The moreover important takeaway from the Glazer family taking dividends while no one else does is the message it displays to sponsors, fans, players, and really anyone involved in the club: The owners do not give two shits about you. Its business first, and sporting results last. Unfortunately dividends impact sporting results in the long and short term, and you can make a simple argument that taking dividends at the current situation is incredibly reckless.
Other club owners invest massively in their clubs and generate income through their extended corporate portfolio by assosciation, not directly removing money from the clubs they own. The reason they dont is that A) It shows they dont need money. Billionaires generally like to flex muscles in their competing market. The Glazers taking money out every year to spread among the Glazer family shareholders first and foremost is indicative of a weak owner who relies on the comparatively small money vs club value that they need to get the cash for their private holdings.
Everyone would be significantly better off with the Glazers attempting to unload to club to Sir Jim Ratcliffe or a collection of partners that are willing to take the clubs in its current financial state.
Lastly, its about tradition and values. Siphoning money from clubs is simply not something football club owners do. Except for ours. Instead of "expecting them to" take dividends, let us rather consider why no one else does. Its certainly not because they are not able to.
When Daniel Ek placed a £2bn bid for Arsenal, Stan Kroenkes answer was "No, I dont need money". - Its a shame the parasites awarding themselvs dividends in our club are not as financially strong.