Explanation
These platforms infer a video's subject from the audio, the on-screen text and the caption, and hashtags are one weak input among those. The practice of appending thirty of them is a habit carried over from an earlier era of Instagram and does nothing except make the caption unreadable.
Where they still help is niche specificity. A handful of tags that accurately describe an unusual subject can assist classification for content the automated systems would otherwise struggle to categorise. Broad tags with hundreds of millions of posts do nothing at all.
The common mistake
Leading the caption with hashtags. It spends the one line that gets read on text that was never meant to be read, which costs far more than the tags contribute.