Interesting viewpoint here. Buffy/Angel spoilers, if you happen to be concerned.
The clearest way to resolve this would be, as Greta suggests, some fundamental limitation to the re-ensoulment magic.
In the case of Spike, this seems fairly obvious. He had to endure a painful ritual to regain his soul back. He, without a soul, had to make a free choice to attempt to earn it back. I really don’t see a great many vampires even considering this, and I got the feeling that the ritual would not have worked if Spikes motivation wasn’t pure- if he was doing it as a means to some evil end, he probably would have failed. Possibly even met his final death.
The Angel situation might be repeatable, in theory. If it wasn’t done, I’d assume there’s a reason with how the magic works. Difficult to replicate material components, the spell needing to be crafted to a specific vampire, something would have had to get in the way. But it might have been interesting to see a subplot where they tried to develop a more generally applicable, more easily repeated version. They wouldn’t need to succeed, or even focus heavily on it, but once in a while, mention it. Maybe when Willow had showed up on Angel, she could have said something like “We haven’t finished, but we’ve made progress- I can do this without <random material component they needed the first time>”. Just to recognize that the issue is there.