Some neutered dogs do. Probably has to do with how mature they were when they were neutered.
A castrated dog has lost only its testicles, right? And while testicles produce sperm, they do not produce prostatic fluid, the fluid that makes up the vast majority of ejaculate (semen).
Compare: A man who has a vasectomy can still "cum." He's just "shooting blanks" (semen without sperm in it).
Unfortunately, cutting off testicles also reduces a key source of the hormone testosterone, whose absence can greatly affect libido, sexual response. That means even though a castrated dog *could* probably ejaculate prostatic fluid, many simply don't. They just don't respond enough to process all the way to ejaculation. They can swell up, appear to make a knot just fine, but... they don't quite get as "invested" in processing to completion. It's there, but "they're" not there.
And of course, some castrated dogs lose sexual responsiveness to the point they just don't want you to touch their cock at all. Pre-castration, "ahhhhh....!" love to be handled. Ruled by hormones. Same dog post-castration, might snarl, might bite. They don't even remember they used to like it.
So it's not a simple "yes they do" or "no they don't" answer. It depends on a number of factors meaning it varies dog to dog.
One 7-8 year-old dane-lab mix we got from the animal shelter was intact when we got him, but part of the adoption contract required us to have him castrated within a given period of time after the adoption. We did enjoy his participation sexually for as long as we were permitted prior to the required castration. Very well hung. Copious ejaculate. A couple months later, not expecting him to be "interested" anymore, we were surprised to see he was still sort of into it. Full hard-on, big knot, and... *surprise!* -- a lot of cum!
It probably had to do with the fact that he was a fully mature dog, very long into adulthood.
Even so, I'd say within a year or so, his interest diminished to the point where he didn't seem to remember what that capability was all about. In other words, don't touch that. It would alarm him and make him nervous and concerned.
No problem for us. At that point, he's just our everything (except sex) companion. Our happy old doofus (afraid of rain, ecstatic about snow). He lived with us for several years before his heart gave out. A very long life for such a large dog.