I have no doubt that for a lot of reviewers the process is extremely wasteful. But for those who make a lot of videos, they're leaving a lot on the table if they don't make an effort to resell their stuff - and depending on where they live they could be leaving a lot on the table if they don't recycle it.

The big ticket items get loaned out to reviewers as part of the company's marketing campaign. Some reviewers are blacklisted from this process (or refuse to be part of it for ethical reasons) and have to beg, borrow, or buy from their contacts in the industry/community.

If a retro reviewer lives in your town, it's possible that they're single handedly keeping multiple secondhand tech stores afloat. Also, if you keep a sharp eye on Craigslist and Facebook Marketplace (easy if you're doing it as part of your job) then you'll see interesting tech appear there literally all the time.

Really prolific reviewers will do regular auctions of things that have piled up. This practice is common but they gotta be careful, as we saw when it got Linus Tech Tips into pretty hot water a few months ago when they sold something they had promised to return.

If you have a production team, then you can have people who spend their time listing and shipping out unneeded items - not to mention that people will tend to just take things that aren't being used if they lie around long enough. On the other hand smaller reviewers tend to get big collections because they run out of people to give things to and selling stuff takes time away from video production.