To be fair, this has been the case forever almost. 70s blockbusters had a distinct feel. 80s, 90s, 2000s, etc.
Even if I didn't know when a movie came out, there are very clear indicators of the era that a blockbuster was made in. And i'm not talking about the tech in the movie or the picture quality.
I think the problem is tonally, aesthetic wise, style of writing are way more homogenized than the other eras. I feel like since 2016 people really wear their influences on their sleeves a lot more in the past. And that kinda makes it seem worse than maybe it is compared to previous decades. Like movies and even some shows don't hide how much they're ripping off Goonies, Guardians of the Galaxy, Avengers, etc. When it the past I felt like even if something was clearly influenced or inspired by something else, they still added their own flair.
IT also doesn't help that Disney is by and large the biggest blockbuster factory.
Playing a little bit loose with the term blockbuster depending on how you define it, but MCU, Star Wars, Pixar, Disney Live Action, Disney Animation all are very successful and are pretty much running things someway or another. I'm not a film historian but I feel like this is the first time that one singular studio had this much power of blockbuster filmmaking since blockbusters became common place.
But I'm sure there was someone in the 90s saying "All blockbusters feel the same" It's all cyclical
EDIT: I also wouldn't be surprised if it's just that we get soooo many blockbusters than we did before that some may feel this way