I think for three reasons:

1. Long COVID isn't a definable disease. That whole ground has been badly polluted by people claiming to have "long COVID" when they haven't ever even tested positive for short COVID, there being no symptoms in common with all reports, etc. It's very hard to say what the long terms effects of COVID really are even though there are now nearly two years of experience with it, for this reason.

2. Long term effects from vaccines have happened before, e.g. early ones gave people polio, more recently there was the Pandemrix / narcolepsy affair. Drugs of any kind are put through difficult safety trials because of a long history of accidents. They are artificial chemicals designed to manipulate the bodies most powerful internal mechanisms after all, no reason why it's impossible to have long term effects.

3. The side effects of COVID vaccines are drastically worse than any normal vaccine. They routinely make people very sick, but it doesn't get treated by scientists as a possible sign of bad things happening because these are "normal" and "expected". Some side effects weren't detected by the trials, like myocarditis, and others weren't detected despite being apparently very common, like stopped periods. Not detected because all the women were on birth control. In fact information on side effects of any kind is extremely poor - you get self reported documentation at best, as there are no major large scale surveys - and the establishment is quite obviously terrified of any attempt to find out more. The trials themselves ignored all events that happened 7 days after vaccination, which doesn't seem very long. That attitude is endemic.

In a situation where all discussion of side effects is heavily penalized or outright erased (e.g. Nicki Minaj losing her Twitter account), it's inevitable that people will conclude something is being frantically swept under the carpet.

Finally, consider something important: the ambient underlying assumption behind the vaccination programme is that everyone will get COVID at some point and it will be the same for everyone regardless of when they get it. In reality it's now been nearly two years and most people either haven't got it yet, even when heavily exposed because they were self-isolating with sick people (I am in this category), or alternatively, got it in such a way that it was so mild they didn't notice at all. If you assume the modellers are wrong again, and that a 100% chance of infection is not in fact correct, or alternatively that by the time you do get it it's mutated to a form that's no worse than a cold, then the tradeoff around vaccines looks quite different even for middle aged people. After all, zero spike proteins is better than some regardless of how you get them.