Because these deviations aren't slight to them.
"Radical" comes from the Latin word "radix" meaning root and usually refers to groups or individuals who consider themselves to have figured out the root cause of a problem and are now trying to solve that rather than proceeding to bother with the symptoms.
So a positive picture of that would be a small boat with a leak and someone closing it with a plug rather than using buckets to remove the water flowing in. A more negative example would be Nazis who'd see a minority as the root cause of all evil and would thus take measures to remove them from the country or from life, rather than to search for means to mitigate or solve the problems differently or at all (most often they are wrong with that anyway).
So there can be quite a difference between radicals and it's rather important what problem they see, what they consider the root and what means they want to take to solve it.
Also as a consequence of that left-wingers are far more likely to self-describe as radicals as they often consider that a positive thing ("Look I've figured it out") and their problems are usually in institutions and structures rather than people, which is usually more palatable, at least in theoretical discourse. While for right wingers it's often enough a minority that is considered the individual culprit, Jews, black people, foreigners, women, young people, progressives, lazy people, entitled people, ... and where discrimination, deportation, coercion or murder are usually "solutions" that are far less palatable and often enough actually illegal. So as a consequence of that they usually try to avoid the "radical" label with a stick, but rather like to present themselves as "centrists" and "mainstream" even if none of their ideas is or should be centrist.
Now the consequence of being radical is usually that you have a clear conception of goals and means to reach these goals, while everything else is either outright bullshit, irrelevant and a waste of time or even counter-productive. Now chances are radicals are less open for result-open-discussions and more in favor of making and negotiating practical demands and often times not even that keen on making compromises as not solving the problem is at least prolonging the problem.
And another rather important factor is the objective or subjective significance of the problem, so while you can technically work yourself up over everything. It's also possible that the subject matter is of major importance or maybe even actually leaves you with no other alternative. Idk if the current course of actions leads to your death or that of a person close to you than change becomes imperative and you get radicalized by the circumstances, though in that case you'd only be radical against something but have a plurality of options to be for something.
So if you then join forces with other people who are against that thing, it can happen that you are against it for VERY different reasons and have VERY different conceptions of what a good alternative would look like or what suitable means would be to achieve that. So despite superficially being on the same side of that issue you very much aren't.
Like idk if you have a corrupt democracy and the opposition is split on being against the "CORRUPT democracy" and being against the "corrupt DEMOCRACY" you might have the same all-caps slogan "STOP CORRUPT DEMOCRACY" but a VERY different implication.
Also for completions sake, given that the media is mostly bothered with the loudest and most disruptive actions, there's a good chance that "radical" is often only used in the context of violent and aggressive actions rather than those that are laser focused and less willing to compromise, also depending on the subject matter there's a good chance of a correlation.
And it's also different from extremism which just means that a group or individual is on the outside/edge and not "center/mainstream". It's possible to be outside of the mainstream because you're radical, but it could also be that you're radical but still in the mainstream or that you're an outsider but not because of radical ideas and as a consequence not all outsiders are alike so using it as a group rather than as a description of a group is often used for fallacious comparisons.