I don't think they expect to do they? They are just using an unlimited bankroll to punish people they perceive as their enemies with hugely expensive and stressful litigation.
Because generally UK judgements can be enforced in the US (and other places).
There are good reasons to expect that in this case they would be unenforceable in the US, but that isn't a guarantee-- and if they are you don't get to go back and fight the cause you lost by default and would have expected to win on the merits. So you're risking a kafkaesque situation where everyone is saying "yep, it's wrong and unfair, now hand over all your assets. thems the rules".
It's also the case that ignoring a judgement in the UK can get you found in contempt and subject to prison should you happen to find yourself (perhaps accidentally) in the UK in the future. Personally I have no problem never going anywhere near the UK in the future, and though "outlaw in the UK" has a nice ring to it, this does add to the incentives.
The person who runs the bitcoin website was sued by the same con for distributing the bitcoin whitepaper, and defaulted because the UK wouldn't allow him to defend himself without divulging his identity. He took the whitepaper down from the UK under threat of arrest (if they ever find him) and now facing hundreds of thousands of pounds of the opposing side's legal fees under threat of potentially losing the domain name (and/or prison, again if they can find him). There really is no silver bullet.
Default judgements can be the basis of internationally-targetted lawsuits aimed at enforcement of same; they can form the basis of arrest warrants, which implies never travelling to places where the UK can send police officers to arrest you, and while it would likely be an amusing basis for someone to start turning e.g. Linux into fiduciary liability for the purpose of legal trolling, who wants to facilitate that?!
To extend the parent question, why can't the people accused, not being in the UK, simply ignore this?