Fail to go back and clear them out, mostly. Most were left open because of something relevant at the time, so I mostly need to spend a little time going through and nuking or nothing. There's been little friction due to leaving them open so it hasn't been a priority.
Pretty much the same thing that leaves some people with tens of thousands of messages in their inboxes (I deal with someone who does that and it makes my teeth itch, so my inbox isn't so bad).
As a programmer who has spent significant time with performance work, having useless tabs in a browser would hurt me. But that Google has to search through a bit longer list of messages I can accept as the typical wastefulness of computing these days. (I am old enough to have done time-sharing on 4 MB with 11 other students on their VT100)
Type '%' (no quotes) followed by parts of the URL or title or both in the URL bar. Searches only your open tabs. I can usually find exactly what I want quite quickly.
I must be quite an outlier, because even 10 tabs open at once sounds like a distressingly high number to me.
I hardly ever have more than 10 tabs open, and aggressively close everything I am not working with. I also shut down my browser twice a day (2 working locations) and never restore the previous session. I do bookmark some pages, but as a matter of fact I notice that I hardly ever refer to my bookmarks. I don't have the feeling that I am missing out on anything.