5G base stations have compliant out-of-band emissions so the issue is not "because of 5G" but rather "because radio-altimeters gather energy from adjacent bands" (something we call "blocking"). Not all radio-altimeters are the same, but for some of them there is very little filtering and the only thing that protected them from good old 4G was that the antenna behaves somewhat as a filter and 2.6 GHz was far enough from 4.2 GHz... It may sound silly for a safety-critical equipment, but we really have to think about the really long lifetime of those equipments and think with the context of the time when they were designed/deployed (e.g. if there is only space-to-earth transmission in adjacent bands, why would they put filters that would add insertion loss and cost ? In the 80's there was no 4G/5G in sight...). Probably the aeronautical authorities also failed to inform the relevant issue in due time within the spectrum regulation process, so this has not been known or studied properly at ITU/CEPT/etc in the context of those frequency bands close to 4.2 GHz. It also seems that the radio-altimeters vendors keep a lot of secrecy on the technical details of their equipments (a part of the internal culture which also comes from the dual civil/military use of those systems) so even people who represent civil aviation authorities or aircraft manufacturers such as boeing/airbus in spectrum regulation authorities do not always have full access to the relevant information.
Now the situation is what it is, and the short-term solution is to have some kind of 5G exclusion zones around airports, and of course operators are not happy with this because they paid huge prices for 5G licences and now they discover that they have new/additional constraints around airports... Long-term solution is also an issue, because it is not that simple to put filters on existing equipments (if they change the behavior, the whole computer need to be re-calibrated, and pilots trained, and the whole thing re-certified, which is also not as simple of doing it once and certifying all aircrafts from the same model but really must be done one-by-one in a custom way for every aircraft). I understood that new specs for radio-altimeters will soon be out so we can hope that the situation improves soon as the retrofit will be able to start (even though it will take years), but there is a possibility that even new radio-altimeters may still continue to be vulnerable to blocking in 4-4.2 GHz in order to maintain their required accuracy (I'm no expert in radars but I trust those people who told me). So the issue will be solved with regards to 5G in 3.4-3.8 GHz as deployed in Europe, but maybe not above 4 GHz (which is one reason why it may be better to restrict to low-power/verticals in the future uses in those bands).
Anyway, this is a complex topic as you see...