My thought was ease of seeing what blew and ease of being able to find a replacement part. Just thinking
The fusible links should only fail (other than age) if there is a wiring short/failure between them and the fuses in the vehicle. The fuses are meant to blow if something down stream shorts out. It's a hierarchy to firstly protect the wiring from going up in smoke and secondly to prevent the device (if the failure is not wiring related) from going up in smoke.
If the vehicle manufacturers thought a fuse would be a better solution they would have put a fuse in place of the fusible link. In cases like this, unless you really understand all the electrical/load aspects of meddling with a proven system, it is a smart choice to leave things as they are.
As mentioned, a fusible link can sustain short high current surges without 'blowing' unlike a fuse. Again, the fusible link is there to protect the wiring harness leading to the fuse panels. It will blow to protect that wiring from melting/catching on fire.
Consider also what happens when something goes really wrong - vehicle accident where part of the harness is pinched and shorted to the body/chassis... These are considerations that are design for my the OEM - most of our after market additions to consider these worst case initiated failures.
couldn't one find some type of "slow blow" fuse? I know these are commonly used on printed circuit boards but unsure whether they make them for the type of amperage that our trucks use? I would think if someone could figure out the specifics of what the fusible link blows at they could find an easy replacement slow blow fuse to put in there. The problem I see with the fusible links is even if they look ok, they can be bad. At least with a fuse it shows when it is blown.