I think for many (me included) it's the same reason many like less clothing rather than more clothing for other sexual activities: the body is beautiful and nice to admire. That's a good compliment to tickling when there is an intended sexual charge to the experience, much like pairing the right wine with a good steak. Both can be good on their own, but some appreciate them together as compliments to each other. Others don't like the taste of one or the other, so naturally avoid doing that.
Tickling is not always sexual for me, or it can be partially sexualized. Whether being sexualized at all, or fully, having more access to more areas does make tickling more fun for me, and often seems to leave the lee feeling more vulnerable and ticklish. At this point, I only do very sexualized tickling with my wife, so tickling while naked is added fun. But when we have a session with someone else, it's either lightly sexually charged, or not at all, but no full nakedness or focus on sexual touch. It just depends upon how the person we're playing with feels.
An example of a sexually charged, but not fully sexualized experience for us: A woman we played with had
extremely ticklish nipples. She and my wife went topless because they both felt comfortable doing so. They both felt more ticklish in the session, and it did make it feel more adventurous and sexually charged, without feeling like anything crossed someone's boundaries, which were all discussed thoroughly up front.
An example of a much less sexually charged experience would be how I love to tickle a girl when she's wearing a sweater. I don't know exactly why, but I guess there's just something about how the material and coverage leads them to feel more secure, and being able to tickle them anyway and feeling them squirm around in it and hearing the wonderful sound of laughter is just sweet fun! Speaking of sweaters and related things, I just wrote a small article where I was processing loosely related thoughts, which may interest you or someone else:
Article link.
I guess I appreciate different elements and compliments to tickling as unique additions to the total experience. Clothing (or lack thereof), scene, ambiance, age, temperature, bondage, personality...everything.
Hope that helps in some way!