Monitor has its heat production spread to wide area, not in one focused spot.Of course; that's half the reason why the capacitors fail. At least they are away from the major sources of heat inside the monitor.
And vertical monitor casing develops some airflow through it with warm air rising out from top and cool air getting in at bottom.
Heatsinks are usually positioned to along vertical airflow.
Compared to that brick PSUs are lot worser with zero airflow and wrapped inside thermal insulation.