Well geopolitics is simply not that simple. What if the Chinese government kept telling the USA that california should hold an independence referendum? Might sound like a stupid example, but Americans would be up in arms at such a proposition. What if China told the USA to end the trade blockade on Cuba so that they fairly take part in the global economy and climb out of poverty? Again, the USA would never consider such a proposition.

The reality is we don't know how many HK residents want to be independent. Ideally they could hold an independence referendum, but the Chinese government don't want that, and due to the geopolitical tensions in the world right now they're not going to take advice from Western nations.

You could make the same argument to the UK about Wales and Scotland, or Spain with Catalunia and the Basque Country. Neither government is going to let a referendum happen for quite a while.