Because it's not all about you. It's about your parents, your friends, your family who are not :gasp: software engineers, and it's nice not having to worry about them going bankrupt or homeless if they need x life-saving medical procedure or they loose their job.
It can be impractical to take your parents, your friends, and your extended family along with you to another country. It's certainly far more difficult than getting a job there as a software engineer, particular because of countries preferring to allow highly skilled, educated, and/or paid people in preferentially.
About 10 years ago I knew a guy who had to turn down an amazing opportunity at another company because one of his kids had a pre-existing condition. This boggled the minds of us non-Americans where healthcare is not linked to your employer. I don't know what it's like today as I have not kept up with what ACA built and what the Republicans have dismantled piece by piece.
I've never heard of a company making pre existing conditions a problem to get health insurance. If you didn't have coverage through your job you had problems, but if your coverage came from you job there generally weren't restrictions like that.
I personally wouldn't want to contribute to such system, simple as that. But also - in (almost all of) EU you're always covered no matter what. You don't have to worry about what happens if you lose your job, or move jobs, or need extra time off work, or accidentally go to the wrong hospital or call the wrong ambulance - it's just not a concern for your average citizen at all. These concerns are obviously worse for immigrants coming to work since their insurance will be very much tied to their employment.
I know enough people who've ended up with long-term illnesses that have put them out of work… and sure, their professional software engineering job provided them with healthcare for a number of months after the employment ended, but then… then they had almost nothing and in all but one case ended up bankrupt.
Every single software job I've seen include several disability insurance offerings for free.
For example, if I can't work, I get 70% of my salary in long-term disability
Does that typically cover healthcare costs as well?