This morning, I had a look at a recent mathematics paper that I am a coauthor of. I was struck by how complex it is. The reasoning in a mathematics paper is extremely elaborate and complex. In philosophy, we tend to think that an argument with, say, twenty steps is very elaborate. But here the proof involves eleven lemmas, each of which has a proof consisting of several, and at times quite a large number of, steps, many of which are quite elaborate. I can see how one can look at a philosophy paper and a mathematics paper, and think: "The mathematics paper, that's really serious intellectual work. The simplicity of even the most complex philosophical arguments, with the exception of ones in philosophical logic, shows the lack of intellectual seriousness of the philosophical enterprise." I think it is not uncommon for scientists and mathematicians to have this attitude towards philosophy.
I think this attitude is mistaken. Anecdotally, writing good mathematics papers is not harder for me than writing good philosophy papers. Writing a mathematics paper takes me significantly longer than writing a philosophy paper. There is a lot more detail. But how long it takes to write a paper is not a good measure of intellectual difficulty or seriousness. Typically (though not always—I think the paper I was looking at is a counterexample) the main difficulty is coming up with the basic idea for the paper. The difficulty in coming up with the basic idea for a good mathematics paper is not very different from that of coming up with the basic idea for a philosophy paper. In both cases, one may spend years thinking about a problem, trying out solutions that fail, and finally the idea may just come—or it may be a series of progressive refinements. Once the idea comes, wrapping it up can be challenging, and in the mathematics case it may involve more tedium (or not—the tedium is different, in the one case there is tedium in getting all the details of the proof right, while in the other case there is a tedium in relating one's result to a vast literature). Of course, one might find that the details aren't as simple as they seemed once one works through them—but this can happen equally in a mathematics and a philosophy case.
Moreover, even in the mathematics case, the length and complexity of a proof is not the mark of intellectual quality. If one could find an elegant, quick proof—that would be all the more appreciated by the community.