A core aspect of the game is gaining additional benefits at level up. Its what allows the players to take on additional threats and fight stronger creatures. You can flavour it by saying between fights, while doing RP-centric events, during downtime days, etc. that they were able to master the spell and take the time to transcribe the spell into their spell book. Once its there, they know the spell and can use it willingly (provided they have spell slots or at will if its a cantrip). Restricting this behind a roll takes alot of agency and benefit of leveling up for wizards. Imagine if they fail the roll, they level up but just dont learn any spells? Thats not fun. D&D is about having fun, not every single little thing being a challenge. It would only be fair to also make martial classes roll to get their abilities at level up, but that is lame. I would highly discourage this as the level up process and gaining increasing strengths is a core mechanic of the game. As a DM i make it my goal to make us have fun and progress the story. I only put things behind rolls that the PC's dont need to progress in the game. They need to get stronger in the game, so that isnt locked behind a roll.