Magic has existed alongside humanity for centuries. Technology might fluctuate, but magic itself remains tried and true, most spells having existed in the same form for thousands of years. While those belonging to different schools of magic often have their disputes, the fact remains that there is no one religion or system that's required to use it, which makes it appealing to many different people. There are almost as many different kins of magic as there are people, so those who seek to learn a specific kind of magic will find limitless options.
However you look at it, magic is essentially based on using your force of will to shape the universe. Magical skill can be inherited, but in this modern age, most practitioners learn it through complicated routes and spells. This is especially true in cities, where the practice of magic has become less spiritual and more ritualistic.
It's possible to have zero magic skill whatsoever, or to create magical effects without properly learning how to channel it. While some may have innate abilities which allow them to use bursts of magic power for a short amount of time, many rely on mana crystals to provide or enhance their magical power. It may have been around for centuries, but that doesn't mean magic has become any easier to understand.