I think it may be person dependent. The recommendations are based on your account, and topics and channels you've watched in the past, or what other people who have watched similar recordings have watched.
NOTE: That is a really crude guess on my part. I do not know the algorithm they use.
I was curious about mine. In a spreadsheet, I typed the run times for the recommended recordings. I did not document all of them, only the first 58 recordings. I also did not document the Live stream/marathons (I'm looking at you MST3K channel). So I have 58 run times running down the left side.
Across the top, I keyed in 00:01:00, 00:10:00, 00:20:00, 00:30:00, 00:40:00, 00:50:00, 01:00:00, 02:00:00, 04:00:00.
Then I created a formulas that counted the number of times that were less than 1 minute, between 1 minute and ten minutes, between ten minutes and 20 minutes, etc, and then those longer then 4 hours.
The results are -
less than 00:01:00: 1
00:01:00 - 00:10:00: 19
00:10:00 - 00:20:00: 17
00:20:00 - 00:30:00: 8
00:30:00 - 00:40:00: 4
00:40:00 - 00:50:00: 2
00:50:00 - 01:00:00: 3
01:00:00 - 02:00:00: 2
02:00:00 - 04:00:00: 1
greater than 04:00:00: 1
The longer ones are either documentaries, Formula One races, or "mini" marathons.
Having done all of that, as lazy as it was because I didn't list ALL times, YouTube videos have gotten longer.
Like blitzkrieg said, it's about making money from the videos. I don't see anything wrong with that. Some people are just really, really lazy.
Here is a tip, and it is possible someone else as posted this elsewhere on Wrong Planet.
If you see a video, or a channel you are not interested it, place your mouse cursor over the title.
To its right will appear three dots.
Left-Click the dots to bring up a menu.
The last three options on this menu are -
Not Interested
Don't recommend channel
Report
I've started using this to block channels I find annoying that have sensational title claims and sensational graphics. I generally give them a fair chance, but if they don't deliver the promised content, they are exiled from my recommendations.