Well for one, I don't think anyone was asking for a 1:1 recreation of a game's plot in the film format. Just something that was faithful enough to the source material.
Secondly, I think you're really underselling one of the franchise's core strengths in the first place: Sonic's basic character appeal
Sonic's a mascot cartoon character, made in the same way Mario, Mickey Mouse, Bugs Bunny, and many other cartoon characters were created. As a result, nearly everything about him, from his looks to his abilities to his character, is all chosen in a way to be as appealing as possible.
They aren't designed with relatability, a sense of normalcy, grounded realism, or any of that in mind; in fact they're specifically outrageous, off-the-wall, and exaggerated nearly 100% of the time. But behind all the crazy is some form of almost inherent appeal, that takes the uncanny and re-brands it into something oddly compelling.
Sonic didn't just become popular because of his gameplay; it was that combined with his character appeal that skyrocketed him into stardom. Even the spinoff media such as the comics and TV shows managed to become massively popular on their own, and those only had Sonic as a character to sell them. And in polls in the late 90's to early 2000's, Sonic was consistently the most popular character in the industry, eclipsing Mario himself. The same Mario that's going to be getting his own movie later on in the year, riding off nothing but his own status and appeal as a mascot character, as well.
You could make the argument that only kids are interested in characters and series like these. That their era has come and gone, and the point of designing intellectual properties around these principles no longer has much merit... but whether that's the case or not, this is, in it's most basic sense, the character's core identity here.
So why on earth should the series' first cinematic debut be attempting to "draw in new fans" from a crowd that would never even think to identify with the character and main franchises' fundamental basis and central premise, while simultaneously trying to hide - if not push away - it's identity as much as it possibly can in the process? Why not capitalize on what we already know works with Sonic, and build from there, drawing in new fans from how good of a Sonic movie it is instead? Sonic isn't exactly some worthless dead-weight character that needs to be propped up here; he was and still is one of the most popular and appealing cartoon characters in the world. They just need to work with that.
"Why you should care about Sonic as a character" isn't answered by having a grounded direction with human characters and realistic tones. That's just the other side of the coin that Sonic was on in the first place, in terms of appeal. And that appeal Sonic specifically identifies with - the zany, out-there kind - is what's there to get people in the seats, in front of the TV, or behind the controller in the first place. The reasons for why you should care about Sonic as a character, if there are any, come afterwards, regardless of the setting.