Why would characters adventure together?

[Shabbazar]

IRL workplaces are pretty diverse... Both of those people can still work at a company on the same team effectively because they want to make money. Adventurers are just co-workers.

Adventurers are not just co-workers. And these aren't just humans of different races raised in the same ultra-tolerant diversity fest 1st world country. Generally, in modern 1st world countries, work doesn't entail regularly facing a real chance of death. I see the enmity between dwarves and elves, let alone half-orcs, lizardmen, etc. as being at least as bad as the worst racial animosity historically seen in humans. In that environment it's kind of absurd to think a lizardman, dwarf, elf, half-orc and a human are all going to team up to face death together.

I think you just need to gloss it over and roll with it. Trying to justify this just seems contrived. It's like trying to explain magic by using physics based real world arguments.