I support open borders and this is one of the points that I've grappled with for quite some time. I'm not convinced that the decision to move from developing country to a developed one is made purely on economic grounds. Permanently relocating your life is not an easy task. You pretty much have to give up all your social capital, all your community ties (which means that you could face ostracisation back in your birthplace), learn a new language, learn new cultural norms, spend significant funds on travel and relocation costs costs and whatever savings you have left will lose a decent chunk of their value due to exchange rate conversion. You also have no networks to exercise in finding employment in your new home and finding accommodation without any kind of reference is going to be difficult.

In that sense, the risk to oneself is so great that only a privileged few in developing countries who wanted to relocate would actually have the resources to overcome them. So for many people, relocating might legally be a choice, but it would be practically impossible.

Realistically, migration is only an option for you if you (1) have a job offer from a company based in the place you intend to migrate, (2) have sufficient funds + skills to cover yourself until you can find a job, (3) have family who can support you and lend you their networks until you can find employment, (4) have sufficient funds to start your own business. Incidentally, those criteria also form categories of US visas (H1-B/TN-1/E-3/L-1, O-1, Family Reunion, EB-5), though, they impose more stringent requirements than economics would.

(Re: your first paragraph) - and yet, we see millions of folks undertaking dangerous journeys over the Middle East or Mediterranean or Central America or Indian Ocean to reach Europe/USA/Australia.... What gives? That life must be very shitty in many parts of the world??

Illegal immigrants from Mexico or Cuba don't have job offers, funds, or family.

Some of them might not, but quite a lot of them have at least one of those; some have all three. It's not uncommon for illegal immigrants from Mexico, particularly, to be people who would be eligible for family-based immigration but for the expense and decade plus backlog. It's also not unheard of for those that have connections in the immigrant community to have an (likely off-the-books because of documentation requirements) job lined up.