It's a cumulative thing too, you can't just blow 400m in one season and expect to win, you generally have to pump money in over a number of seasons, and even then, just spending as much as Liverpool / United only gives you an outside chance. To have any genuine confidence, you have to outspend them considerably, which is probably why very few even try.
It's just not financially viable to blow hundreds of millions on players to win the league if you're hoping to actually turn a profit in the process. You simply won't manage it. It's better to spend a modest amount (enough to stay in the PL). If you don't necessarily have to make a profit (like us) and you've money to burn, then like us, you can do it. We've been fortunate enough to tackle the problem on two fronts though - by spending a fortune on players AND on infrastructure to support us in the longer term. Hardly any other clubs can do that with the same gusto as us.