First time enlistees are absolutely not given the option of whatever job they want ex ante unless they ask/push. Many/most are not negotiating up front. Maybe truck driver was a bad example but there are many jobs that either require many people to fill them so recruiters push or significant screening so they are unrealistic posts for most.
Recruiters are known to have quotas and push hard. You think your average US Army Recruit manages this morass well?
https://www.goarmy.com/careers-and-jobs/browse-jobs?position...
Even weirder that HN expects the average joe to “get it in writing and check the fine print.” A good number of recruits are not US citizens and may not speak english super well. Some 35k+ active duty and 5k+ added a year for the research inclined.
Even without a written guarantee, in volunteer forces the senior officers will generally get rid of new recruits who decide they don't want to be there instead of forcing them into a different job. Especially for the combat jobs where even training can be deadly. Better to give them discharge papers and GTFO rather than wasting money training someone who's going to be a poor performer with constant morale and discipline problems.
(Of course if there's ever another world war and conscription is reinstated then the rules will go out the window. But that's not what we're discussing here.)