Bards are always stealing secrets, once they won a case where Tasha, Otto, Leomund, and Mordenkainen had trouble establishing copyright, they added those spells to the general bard list.
It's a well known fact that sorcerers have self-esteem issues and so like to take the spells of wizards and show them how they can cast them better in the hopes of some small glimmer of approval.
Everyone knows that Tasha and Evard came up with their spells after watching an Old One who once met them and went "Hey, look what I can do!"
Eldritch knights and arcane tricksters are part of the wizard club so they're allowed to have access. They're like miniature wizards who don't have the focus to prepare spells and instead have to focus on a small handful of spells.
Melf let acid arrow to swamp druids after they lamented the lack of an appropriate spell of second level. He's still collecting royalties.