Are there other reasons than "the world is an illusion or a simulation" that the radical skeptics use to doubt almost everything? I don't think there's a better reason than it, but maybe there are other reasons to doubt a lot of things, what are some of them? In my knowledge there aren't other reason, but I might be wrong.
The fundamental argument for freedom of opinion is the doubtfulness of all our belief.
The fundamental cause of the trouble is that in the modern world the stupid are cocksure while the intelligent are full of doubt.
So if you don't want to be a radical skeptic, there's at least some philosophers' quote to argue for some skepticism in general. In deductive reasoning like logic and math realms, most common knowledge status quo may be hard to doubt under their corresponding applicable contexts or axiomatic systems. However, in inductive reasoning realm the confirmation of an argumentative conclusion in the form of hypothesis is very susceptible for doubt due to its limited background assumption and knowledge. For example, I. J. Good gives an example of the observation of a black raven actually decreases the probability of the to be confirmed hypothesis "All ravens are black" according to here:
Suppose that we know we are in one or other of two worlds, and the hypothesis, H, under consideration is that all the ravens in our world are black. We know in advance that in one world there are a hundred black ravens, no non-black ravens, and a million other birds; and that in the other world there are a thousand black ravens, one white raven, and a million other birds. A bird is selected equiprobably at random from all the birds in our world. It turns out to be a black raven. This is strong evidence ... that we are in the second world, wherein not all ravens are black...
So even you did some effort and empirically confirmed a hypothesis per your specific limited observations, you cannot claim such confirmation is strong enough or even relevant at all to inductively infer the intended hypothesis in some cases without knowing all other background assumptions, hypotheses and knowledges which may be never knowable as above example exemplifies...
Famously, Descartes assumed that a claim was false if it "falls prey to even the slightest doubt". He reasoned that if he did this, than any conclusions that he reaches must be undeniably true. Very few people claim that he was incorrect in this idea. Instead, people who reject his philosophy tend to claim that his notion of 'doubt' was not strict enough or that he made errors in his formal logic.