Sovereignty works not only for full democracies, but also for monarchies, dictatorships, and various kinds of representative decisionmaking.

But the main point I was trying to make is that if "you were born in England and lived there all your life. Then you fly over to Thailand on a tourist visa" then whatever social contract Thailand people have established in Thailand doesn't matter - you are not entitled to its benefits, you are not part of their social group (yet?), you don't have an inherent right to be part of that group, and it's up to them whether they will extend their social contract to you at all and if so, on what conditions. In this regard it doesn't make a meaningful difference how Thailand decides it and who gets asked. Self-determination of countries is a thing even if those countries are internally governed very unfairly - coming from outside it doesn't matter, you anyway wouldn't get a say on how they're governed.

Again, it doesn't matter of all or most people there were asked on that - if they don't like their system, they're free to change it, but until then whoever or whatever rules there gets to make the rules how Thailand will handle travel across it's borders.

Unrestricted travel is not a core human right, the right of national/regional self-determination is considered more important and overrides it. There is a right to decide who visits in your territory, and there is no inherent right for a visitor to come if they're not invited.