Well, for starters, there are many points of the process where your entire future is down to the mood of the immigration officer at the airport. Making it so that the safest way to not have to potentially uproot your entire life in hours is to just keep your head down and be a good little indentured servant until you get lucky in a lottery.

Happened to travel internationally, either to see family for a week or for a conference in the 3 years between finishing your education and getting a chance at an H1B? Maybe they'll let you back in, maybe they won't, no one can say.

Happened to study and work here for over a decade, slaved away for an employer to convince them to sponsor your residency?, Employer screws up your green card application after pushing it off till the H1B limit, you get screwed and have to leave with zero recourse.

It's no wonder illegal immigration is such a big issue here, especially with sanctuary cities, you come and stay illegally and governments will bend over backwards to help you stay, you come here, get an advanced education and work hard to remain in legal status for a significant chunk of your life, supporting yourself financially and giving back to the country and you get kicked out for something that wasn't even your fault.