The reason is possibly that eBay tries to force sellers to use their Global Shipping program, which is sold as a great benefit to sellers, but is an absolute ripoff for buyers. For a seller, you ship the item to a location in the USA and so long as it arrives there, you're golden.
For the buyer, the shipping costs aren't disclosed, and they are very high. A 1kg item could easily cost $50-80USD to ship to mainland Europe. eBay constantly tries to turn on the global shipping option, and as soon as it does, it's difficult to get out of it.
Anyway, many sellers don't want to deal with it, so perhaps list items in the USA only and ask buyers to contact them for international.
A note to buyers - using a reshipper to buy items from sellers who say they don't ship outside the USA is screwing over those sellers. Buyers generally know they are giving up their protection of "item not received" when using a reshipper, so those who do it always just claim the box was empty, or the item was significantly not as described, or was counterfeit or something similar. Then the seller is on the hook to get it back and is out the money. It also eliminates the seller's ability to ship using signature delivery. And reshippers are often notoriously bad for hidden fees and crap service. So when things go wrong, buyers are quick to just do a chargeback or file a claim - which stiffs the seller who had nothing to do with any of that. I'm dealing with two such situation now. A buyer who bought a camera using a reshipper that was not allowed to export to his country. He ignored the auction listing where I said no reshippers, and it's been nothing but a hassle trying to cancel the auction and get my fees back and he's acting like a petulant child over it. Another that bid on a Playstation VR headset that was listed that it would be sent signature required. Of course, he wasn't there to sign for it since he lives in another country - so it's been sitting at the post office and I won't get it back until it's been 30+ days. At some point, he will file a claim and I will be out the $40+ shipping, because this asshole didn't read or didn't care that I stated US sales only, no reshippers.
So please, if you intend to use a reshipper, ask the seller if they are OK with it first - or at least accept that the sellers is only responsible for the item when it gets to the reshipper, and you take on additional liability for shipping the item back to the USA, even if it wasn't as described or was missing parts or whatever else.