"I can't make something like that"
Yes you can. Here's a brief look at a channel that just made some steady improvements over time. Are they the best thumbnails? objectively no. but they are better than they were and you can see that it paid off for this YouTuber with pretty exponential gains - so what's better in them?
Better lighting. Paying attention to the rule of thirds, contrast in the text, centered focal points, being real, some slight editing, and also the titles are a smidge better as well.
Hope this was helpful!