People in the developing world can't just get a tourist visa to the UK (or EU). It's a lengthy application process where you have to provide all sorts of evidence that you can afford it and that you have a permanent job to return to. I remember going on holiday to Spain, Italy and France with my Indian friend, and it was a right pain in the arse to sort out. Or saying "let's go to Thailand next week" only to realise that our Russian friend couldn't get a visa on arrival like the rest of us. Being able to just buy a flight and turn up in a country is a privilege that most people in the world don't have.
Every country has the same problem because no country actually wants to process asylum seekers. They all talk a good game while secretly making it as difficult as possible. Hence my race-to-the-bottom comment.