Question: Why can't I see my tokens after withdrawing from Binance Smart Chain (BSC)?
Answer: We've learned that Binance does not automatically withdraw to the main Ethereum network by default. They allows users to select Binance Chain or the Binance Smart Chain, their own blockchains, which allows Ethereum-based tokens to be held in the form of BEP2 and BEP20 respectively.
Question: How can I fix it if I already transferred my tokens from Binance?
Answer: If you already withdrew your tokens to their blockchain, you'll need to use the MetaMask add Custom Network feature to add the Binance Chain or Binance Smart Chain's RPC URLs endpoints to your MetaMask. We do not have a hosted or recommended URL for a Binance Chain node, you should probably refer to their documentation to find a URL recommended by them.
Once you've added the Binance Chain or the Binance Smart Chain to your MetaMask, you will be able to select different networks to view the assets (you may need to add Custom Tokens too) held by your selected account on that network.
Question: What to do to avoid this and transfer directly to the Main Ethereum network?
Answer: First of all, for the simplest experience with future withdrawals, you can select your Transfer Network on Binance to be the "Ethereum (ERC20)" option, which will allow your assets to be easily viewed on MetaMask.
To transfer those assets from the Binance chain to the main Ethereum main network, you can use their Binance Bridge site. You will need to have some BSC tokens to pay the transaction fee. It shouldn't take too much.
Eventually, MetaMask looks forward to making it easier for you to view assets from various networks all at once, but for now, it's good to understand where your assets are really controlled and make your own decisions about where to withdraw them.