Your downloads folder is full of MKV and your timeline pretends it does not exist. Several big editors still refuse MKV outright, so you convert before editing. It is a product decision, not a technical limit.
- Shotcut: opens MKV without complaint. - Kdenlive: same, open-source and friendly with MKV. - HandBrake: convert MKV to MP4 first; one extra step beats fighting the timeline. - Shutter Encoder: same idea, more control. - FFmpeg: the one-liner that does it all, for terminal people. - CloudConvert: the browser option when you cannot install anything.
MKV support is a good proxy for how much an editor respects real-world workflows. If a tool cannot open one of the most common containers on earth, what else is it picky about? And for quick image format conversions on the side, PicConvert is free forever.