I think you'll find that it's mainly because you're talking nonsense. The UK may be considered a country, but the 5 teams you're talking about represent THEIR country. Yes, the UK may be a country, but so are the others you're denigrating.
england's international failure lies with the PL, because their league mostly consists of non-english players. You have a much smaller pool to choose from than you've ever had. And it's going to drop 20% if this new proposal is adopted.
Personally, I'd much rather follow a **** Scotland team than a highly successful UK team (for UK, read england). I think the vast majority from the countries you've denigrated would agree.
I "denigrate" no-one. England, Scotland, Wales etc are only "countries" in a football and rugby sense. Nothing else.
Do you doubt that the UK would have more international success in football if it entered just one team ?
The better players migrate to the better teams. And contrary to what you might think, EPL teams want to sign English players, why on Earth do you think they're so expensive ?
Would Manchester United have paid 85 million for Harry Maguire if he wasn't English ? Absolutely not.
Your parochial attitude is why Scotland are so poor. Whist you think of yourself as from a small country, your thinking will always be characterized as small minded.
Why stop there ? Why not follow an even poorer Glasgow or Edinburgh team ?