Druids in 5e don't necessarily have to worship, but a lot do worship various nature deities, same goes for rangers. Paladins can now swear themselves to causes, rather than gods, but again, many still worship gods. Its about giving people options to play the kind of character they want. Also it is a side effect of getting rid of divine champions and blackguards as classes, paladin had to broaden its class portfolio. There's absolutely nothing stopping you from going old-school with your paladin, but you have options.
It's why I think everyone should have the option to select a god, because lots of people still make characters that are religious, even if they aren't playing a cleric, or need it for their class.
