As already mentioned: availability, convenience, and cost are huge.
It's also less pressure, a more comfortable environment (home vs. stranger's office), no commitments to a next session, and less embarrassing (sharing your personal issues to a computer via text is less anxiety-inducing than saying them to a person's face).
With that all said, I'm strongly opposed to people using LLMs as therapists.
"Sharing my data" isn't something that cross the average person's mind unless there is a checkbox they annoyingly have to check in order to check in to some European hotel which is asking you for permission to process their data, for better or worse.
In their mind, most of the times, if there is no one standing behind them when they chat with a LLM, then the conversation for most intents and purposes is private.
Obviously, us who are born with a keyboard in front of our hands, know this to not be true, and know we're being tracked constantly with our data being sold to the highest bidder. But the typical person have more or less zero concerns about this, which is why it's not a priority issue to be solved.
Finding a licensed therapist, especially one covered by health insurance, who takes new patients, can be a challenge in some areas. So while it obviously is a bad idea, I can hardly blame people in a bad place looking for at least some help.
Availability.
There must be many other reasons, but IMHO that has to be the biggest factor. Being able to just start a session, in the moment, when you feel like it, is a fundamental difference.
Counter-intuitively, I think the fact that it's not a human seems to have a non-negligible effect too. It's a computer program you can share whatever with, and it'll never judge you, because it cannot. It reads exactly what you write and assumes you're faithfully answering, then provides a reply based on that.
I haven't been so unlucky myself, but I know many who've had terrible first experiences with therapists and psychologists, where I'm wondering why those people even are in the job they are, but some of them got so turned off they stopped trying to find anyone else to help them, because they think most mental health professionals would be the same as the first person they sought help from.
My experience is that they (at least copilot) are at least on par, if not better, than self help books. I assume they will get better over time.
Just my few cents
I believe that 'help or not' is a question impossible to answer, objectively at least. My subjective answer is that psychologic help in general is effective, whether it is provided orally, via books or as recently, via automation.
People use opiates as a replacement for mental health providers for similar reasons. While I’m all for harm reduction, it doesn’t mean we should view it as inevitable.
See also, all the ads for prescription medication on TV. Maybe it's just the programs I watch but it really seems like this has become the predominant advertising. Every break has an ad (or several) urging me to "ask my doctor about..."
Should be banned. Average people have no basis to know whether drug X is appropriate for them. If your doctor thinks you need it, he'll tell you. These ads also perpetuate the harmful idea that there's a pill for everything.