That's because
SPIDER-MAN is basically the only that's HAD his own show or least 50% of Marvel's shows in contemporary times. You can't guest star on your own show can you? But if Marvel would let anyone do it it would be Wolverine and Spider-man

. Let's see Spidey's had since the 80s;
SPIDER-MAN AMAZING FRIENDS
SPIDER-MAN (early 80s)
SPIDER-MAN (90s)
SPIDER-MAN UNLIMITED
SPIDER-MAN MTV
SPECTACULAR SPIDER-MAN
ULTIMATE SPIDER-MAN
I feel like I'm missing one but that's basically half of Marvel's shows in the last 30 years.
Captain America, Thor, Daredevil in that time ZERO? So if I'm a Spider-man fan I'd hardly feel slighted or dare I say UNDERexposed. Now a Cap, Thor, and especially a Daredevil fan (who doesn't even guest) I can listen to ***** all day. HELL (no pun intended), I'm one of them (Cap being my fav).
But another very annoying and real reason why Spidey hasn't guested anywhere outside his own Spidey-verse was that blasted Sony character rights legalities preventing him or his rogues from appearing in NON-Sony ventures. Not sure where it stands now but I
think it's expired and Marvel has total dominion over Spider-man.
And if you notice since the 90s Semper series (USM aside) not a single hero guest star has appeared on SUL, SSM, or MTV SM. Goes both ways, I for one thought
SPEC SPID was sorely lacking a few guesties. Might of made me even tolerate the show despite those 6 year old eye-sore char designs I couldn't look at without getting a migraine