Harassment and fraud is illegal, but people do illegal things all the time.
"court they can't get a judgment against you without you knowing about it"
... in theory, but in practice there are several ways to "serve notice" that don't involve the other party actually receiving the notice. Sure, if you can hire an attorney you may be able to get the default judgement overturned if you find out about it later, but again, it isn't all roses.
I'm not an expert on every states laws, but I don't think that the standard of proof for an uncontested claim isn't particularly high in most jurisdictions.
And to add to this, if a debtor doesn't get served (let's say that they took some bad advice from a friend and decided to dodge the constable trying to serve them in the hopes that "since I wasn't served, I can't be sued" is accurate), the constable will notify the attorney that contracted them to serve papers, saying that they couldn't serve them in person. At that point, the attorney sends a certified summons to the debtor's last known address and is then allowed to go to the court clerk and say that they've tried their best x number of times but the debtor can't be reached. They will enter their certified proof of mail into the record and the case will be tried on the scheduled date regardless, resulting in a default judgment when the debtor doesn't show up. Once that happens, if the debtor is employed (and even depending on the type of debt), they'll get a nasty surprise when their employer notifies them that their wages are being garnished and that there's no longer anything they can do.
"court they can't get a judgment against you without you knowing about it"
... in theory, but in practice there are several ways to "serve notice" that don't involve the other party actually receiving the notice. Sure, if you can hire an attorney you may be able to get the default judgement overturned if you find out about it later, but again, it isn't all roses.
I'm not an expert on every states laws, but I don't think that the standard of proof for an uncontested claim isn't particularly high in most jurisdictions.