Why did the colonial empires collapse after World War 2?

It didn't, it just slowly reduced.

A number of countries still have what are in essence, colonies.

Of the main colonial powers Britain, who had the largest colonies, has a small number it still runs. France has a few but gives them the status of provinces of mainland France even though they may be thousands of miles away. The Netherlands did the same with some of theirs. The USA still has a few, one of which, Hawaii, was incorporated into the USA in a similar way to France's colonies.

Most European colonies in the Far East were influenced by Japan's takeover during WW2 and soon fought for their independence shortly after the war ended.

Large colonies such as India were considered too much for a greatly weakened Britain to hold on to after the war and Canada, Australia and New Zealand had been maturing as separate entities for some time and it was only a matter of time before they were truly independent.