Teleport, a 5th level wizard spell, cannot be used offensively. Planeshift, a 5th level cleric spell, can be, if you send someone to the plane of fire.
I'm curious if anyone here has any insight as to why clerics, who never get teleport except for the odd domain, get to travel to different planes more easily than sorcerers and wizards.
"Insight?" Not per se. My guess would be (and personal rationale has always been) because Clerics are the "dealing with other planes" guys, in general. Their whole magic shtick is about contacting/receiving power from other (generally, the "higher" or "lower") planes.
Makes sense Clerics get plane shift a little earlier. And, as you noted, the arcane "teleport" is a 5th level spell. So wizards are getting their "bop around at will" thing at the same level. It's just different/more difficult for them to get to another plane...and it's more "difficult" for clerics to get from one place to another on their plane.
Recall, also, that Clerics get "Word of Recall" which is, essentially, a personal/defensive teleport that comes in at 6th level...so they're kind of evenly matched...though WoR has a single stipulated destination, you can still take whomever you're touching...which might account for its lower level than when mage's get Plane Shift.
I'm also not entirely sure Plane Shift has useful "offensive" capabilities, seeing as the need, as with Teleport and WoR, you need to be touching those who the spell effects, and
you go with them! So, sure, if you're immune to fire damage, you can take someone and "drop them off" there. But it's not like Banishment or Maze.