...it's also an almost perfectly calibrated torture.
You have the prospect of the thing hanging over you, like an iceberg moving your way that you can't avoid, unable to make long term plans because you have little idea what your schedule will be like after the next scheduled event. Maybe in 6 months it'll be over, maybe you'll be in the middle of a trial, or depositions, or who knows what.
There are huge delays where you can do little productive, waiting out a court decision or for your oppositions filing clock to run out. It's easy to fall into a trap of spending your time worrying if there is something you missed. And it can be difficult to adopt other activities to take your mind off it since you don't control your future schedule, and those activities might end up as evidence in the case particularly if the lawsuit is over something that is your profession or your passion[1].
Then you have sudden panics as unexpected things come up with short deadlines. Then back to the wait.
Third parties often don't get why being personally targeted by litigation, particularly frivolous litigation, is such an imposition. Sure the specific hours spent handling things are easily understood, but the inability to make plans and the psychic cost of living under threat are less easily understood by someone who hasn't been there themselves.
People are prone to discount the impact proportional to the frivolity of the case, but the legal system is adversarial-- you must fully and competently defend even a fairly frivolous case or you will lose. I think the case being entirely without merit in some sense makes the pressure worse, since your opposition can make moves to strategically impose on you as they had little to no prospect of winning unless you screw up, even if those moves ultimately make them more likely to fail.
A well founded case can be clean with few distractions, you set out the fact-- the court rules. Or doesn't even rule because once the facts are established you can reach a fair settlement. With a bullshit case, potentially anything could happen.
Perhaps some people have dispositions that handle having their neck on a chopping block for years without trouble. But I know people who have thrown cases they were as close to guaranteed to win as any simply because they couldn't handle the stress of it and would rather lose and have it be over.
[1] To give a concrete example, I was being sued for several billion dollars in relation to a volunteer open source project that I hadn't contributed to for a couple years. I made a pull request to an unrelated piece of open source software in an unrelated field (a driver for a telescope focuser motor) only to have that show up in my opponents filings as "evidence" that I was still contributing to the project at issue. This is a stupid claim, but it's a question of fact, and potentially enough to keep me from being dropped from the case on a summary basis.