These clubs have the resources to keep improving their teams without losing players they don't want to go. Our open door policy and lack of spending power means we'll struggle to maintain our current (Not CL winner level) squad let alone improve it, whereas they can realistically improve their squads to the point of being competitive in the latter stages.
Yes they can transform a mediocre squad into a top squad in a window or two because of their ability to buy and keep top players in their teams. However, TIME to gel and gain experience in Europe and to navigate tough knockout games is a key factor in winning the CL.
Madrid always had an insane starting 11 ever since Pellegrini was in charge. But it took a lot of time for them to gain the right mental fortitude to be able to win it. The same goes for PSG. Has had a top squad for years now but its taking time to develop mentally as a club before they can win it. Signing experienced winners like Alves and Neymar might give them that extra confidence push so they dont crap their pants. Bayern had to go through the same process of mental growth and we are still in it. The main reason we lost these finals is mental more than squad quality on paper.
We certainly have an advantage of having experienced 2 finals, and have always made it past the last 16 for a while now. Thats our advantage. We are used to the big stage we just need to take that to the final. The EPL teams never went through that and it will take them a few years before any of them is a competitor for the CL title. City took ages to just get out of the group stage.
ATM is the only team that I know of that suddenly emerged as a serious contender right after Europa League. Every one else took a few years of build up.