
JDLPF |

To answer the OP, as others have said, it's a hold-over from old D&D lore about the demons and devils.
The first demon monster entry appeared in Eldritch Wizardry by Gary Gygax in 1976, the 3rd supplement for the original Dungeons and Dragons box set from 1974.
From the entry on page 29, all demons gained the following abilities:
•Infravision
•The ability to teleport (no chance of error)
•cause darkness (in varying degree of effectiveness)
•and open a "gate" (varying degree of success according to type)
Monster Manual I for AD&D 1st edition by Gary Gygax in 1977 gave us the addition of devils, as well much of the basic planar lore we continue to see used today.
From the entry on page 20, all devils gained the following abilities:
•Charm Person
•Suggestion
•Illusion
•Infravision
•Teleportation (No Error)
•Know Alignment
•Cause Fear
•Animate Dead
Ever since original D&D demons (and later, devils) featured teleportation. Pathfinder developed from 3rd edition, the edition which added the 50 lbs. limit on transported items. As for the question of why they can teleport in the first place, you'd have needed to ask Gary Gygax, but he sadly passed away in 2008.
If you could summon something that can cast teleport an unlimited times per day, somebody would use it to move a lot of stuff around. Limiting demons and devils to 50 lbs is to prevent them from becoming a cheap teleportation mule.
Note that the text of the Summon Monster spell says the following:
"A summoned monster cannot summon or otherwise conjure another creature, nor can it use any teleportation or planar travel abilities."
You'd have to use a calling spell like Planar Ally or Planar Binding to conjure up an outsider that can teleport stuff for you. Summon Monster versions can't use these abilities.

JDLPF |
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Also, if you accept 'official' modules as canonical lore, then there is a 2nd edition Planescape adventure which explains the fluff behind the abilities of demons and devils to teleport at will.
The Maeldur is the living focus of a teleportational matrix that can open paths through reality for any creature whose name it knows if they subconsciously focus on their destination. The yugoloth constantly whisper the names of every created demon and devil to this creature, granting them the ability to teleport at will.