In theory, I agree. The PHB really should have a sidebar about "creating the party" right there in the "creating a character" section, discussing things like choosing varied classes for the party, and the importance of the characters being able to work together. The DMG should provide some solid advice on how to bring parties together, hook them into the adventure, and make them care about the setting. And the published adventures should provide good examples of this. What we have is pretty half-assed.
However, I kinda think that this is an area where maybe it's okay to half-ass it. Honestly, I don't think there's really much value in agonizing over
why the PCs work together and trust one another -
the important thing is that the players agree that they do, and then you can throw on a wafer-thin justification for it.
If you do want more, way back in the mists of time Dragon magazine published an article entitled "Keeping the Party Going" (issue 177). It gave some possible examples:
- The deocentric party - all the characters follow a common deity.
- The town party - all the characters come from the same town, which is now threatened.
- The familial party - the characters are all related.
- The mercenary party - the characters are mercenaries who have been hired to do a job.
- The guild party - the characters are all members of the same guild (whether thieves or otherwise).
- The quest party - for whatever reason, the characters have all sworn to complete a common quest.
- The clan party - like the familial party, but a wider group.

- The chance-meeting party - because why not risk your life with random strangers?