On top of that, if you're burned out, then you aren't acting at the level of a senior engineer. These titles matter, you're being paid for your seniority.
The forces behind burnout don't decrease just by operating as a senior.
My point is that seniority implies a certain level of professionalism. If you're getting burned out you're not at that level.
I infer that you mean that a senior professional is more adept at mitigating the effects of burnout. While that may often correlate, I don't think it follows that someone is less professional or at an insufficient level if they are suffering from burnout. Many times the factors at play exist within an organization regardless of how one member performs.
The extension of your worldview is that a General in the military can't get PTSD because they're so professional.
I think if a general had bad enough PTSD that it was negatively impacting their day-to-day job performance they would be demoted from being a general.
I don't think that burnout is a function of seniority or lack thereof.
Many things contribute to burnout, including poor team fit. Sometimes circumstances mean that a professional needs to take a chance on a job; other times circumstances evolve beyond what a job appeared to be.
Perhaps consider that a "Senior" Product Manager at a mid-size company doesn't have the same perspectives of actual engineers at much larger companies.