> If someone sues you and you don't respond, you risk a default judgment against you.
There are entire companies set up with the purpose of suing people who don't respond. They purposefully use bad law to operate these mills, and absolutely no one seems to actually care. Google pocket filing[1] if you want to see those that abuse this the most.
In my state serving someone (for small claims) is simply tossing a first class letter in the mail (no tracking or registration required) to the "last known" address. Well, last known address is trivial to game - just pull a credit report send that letter to an address you know they no longer live at and attest to the judge you did your best. Or send them something that looks very much like spam mail, so hopefully they don't even open it and toss it into the trash.
When you create massive incentives to cheat what outcome do you expect to get? Legally serving someone without them knowing is trivial, and I'd argue becoming the normal default behavior for petty litigants like debt collection lawsuit mills.
There are zillions of ways around process serving, from petty little games like the above all the way down to hiring corrupt process servers. The latter almost always get away with it, and I've literally recorded them tossing paperwork in the trash without ever knocking on my door while attesting to the court I was properly served. The court seemed to think this was completely normal and didn't blink an eye.
Basically process serving is entirely broken and relies on the plaintiff serving you being an 'officer of the court' who would never even think about doing anything illegal or immoral! Literally that is the sole protection most defendants get - a piece of paper submitted to the court that someone really really totally served you - honest, and they are double-dog dare you not lying, promise! This piece of paper is treated as gospel by judges, even when you have video evidence showing otherwise.
My few experiences with the American legal system basically have shown me the entire process is designed around corruption and getting "free" judgments outside of court by intimidation/shady tactics around having you lose on a technicality before you even show up. Once in court if you have money to defend yourself you generally do alright - but good luck getting there when the entire system is designed to keep you out of court and ensure you are at a disadvantage before you even start.
1: http://caveatemptorblog.com/647/minnesota-is-a-friendly-stat...