Issues converting MKV to MP4

I just finished converting an animated movie from MKV to MP4 using "Adobe video converter," but now the audio and video are completely out of sync—it’s a total mess
Does anyone happen to know what might be causing this issue, or perhaps you could suggest a more reliable converter?

I honestly can’t believe it—on a colleague's suggestion, I simply swapped the extension from mkv to mp4 and now it plays perfectly on my TV (even the audio sync is spot on).
It makes me wonder why anyone bothers with those clunky conversion programs—which, let’s be honest, can drag on for an hour or two—when just renaming the file solves the entire headache in a single second.

Look, at the end of the day, an MKV is basically just a container—it’s like a digital box where you toss the video, the audio, the subtitles, and all that extra junk together.
It’s not even about what’s inside, because back in the day, you could have had a movie sitting in an .avi format tucked right inside one too, for example.

I’m still struggling to wrap my head around the logic here—what is even the point of these MKV to MP4 (or MKV to AVI) converters if you can just manually rename the file extension? If all I need to do is swap the suffix so my device recognizes the movie, then why do these tools exist at all?
Honestly, I feel like I’ve been throwing away precious hours of my life—sometimes an hour or two per movie—converting files that would have worked perfectly fine with a simple rename.