Is there any reason why the engineers from poorer countries will emigrate to France instead of the better-paying countries?

Romanian programmer here (I still live in Romania), I would choose another European country over a well-better paid job in the US in almost all cases, the reason being that that way I'd be closer to home, we actually have decent healthcare in Europe and the social contract as a whole is more robust compared to the US (it mentally helps going out on the street and not seeing the multitude of homeless people that I've read one can find on the streets of San Francisco), public transport is decent to exceptional and for me personally there's also the gun-control issue (or lack of it, more exactly), I really don't see myself living long-term in a country as gun-loving as the US is.

San Francisco’s homelessness situation is an exception, certainly not the rule. It’s an entire industry in SF with a turnover of $xxx million.

Other cities in the US are much more in line with the big EU cities in that regard.

I think it’s UK/Germany vs France rather than US vs France

Have you been to any major French city recently? There is still a crisis of homeless refugees, there is clearly different reasoning behind why they are there but in general I'd argue the effect is the same.

1) Closer to home country. 2) No visa needed. 3) Less anti-immigrant rhetoric. 4) Better work/life balance. 5) Free healthcare.

If 10 years ago there were predominantly outfits servicing larger US/UK/Scandinavia companies, now it's shifting towards building their own products/services. Basically the guys learned how to do it and not they start spreading the wings. The only thing missing in Europe is active and mature VC sector that could match that in the US.

Less anti-immigrant rhetoric... In France?

Compared t the US? Sure. And not from the government itself.

I didn't say there's none. Every single country in Europe has varying levels of it but nowhere near to what the US have been having over the last few years.

I don't know, I think you have to look at it with an EU scope. Within Shengen only Luxemburg and Switzerland pay significantly better than France. Germany is more or less aligned