Why would people not feel comfortable modifying GPL code? The entire goal of the license is to allow people to make and distribute modified versions.

Because you're required to participate in some sort of re-distribution activity that is unrelated to your primary goal.

That isn't true. All the GPL requires is that if you distribute a modified version of something licensed under it, you must also make your modifications available to users under the same terms. If you don't distribute your modified version, the license doesn't apply to you at all.

From the perspective of someone contributing code to an open source project, there is no difference between a MIT-licensed or GPL-licensed project -- your contributions are covered under the same license as the project as a whole.

OP specifically cited companies that were wary of the license requirements with respect to code modifications. Not sure how it can get much clearer. The licenses are not even similar.

I just don't see where "comfort" comes into it, unless it refers to people wanting to release proprietary modified versions or extensions. If that's the case, "please change your license, because I would like to distribute proprietary derivations of your software" is a different argument than "Please change your license, because I do not feel comfortable modifying GPL code". The latter makes it sound like GPL code has copyleft cooties that jump out of the code and onto developers.

Yes, that's exactly the baggage that GPL carries.