To me the biggest difference is how bitcoins are created. It takes a great deal of investment in time, effort, money and energy just to mine and produce a single bitcoin.
Unlike altcoins, you can't just print a bitcoin out of thin air whenever you want.
What are other differences?
Many differences between them.
You forgot that altcoins can have Proof of Work algorithm too, so with PoW altcoins, there were not created from thin air. A big risk is premined PoW coins, you must check this information of PoW altcoins.
Bitcoin is unique by having many following things: PoW, no premine, decentralized network, no censorship, and decentralized developments so no issue if one Bitcoin developer left like how Satoshi Nakamoto left the community and no longer developed Bitcoin Core years ago.
Altcoins are centralized, mass premined coins, hidden premined coins, mint from thin air with smart contracts, centralized, censorship, can die any time because development is centralized in founders and core team members.