You need to manually add the custom token address of WETH to Metamask.

Open Metamask and scroll down to click on add tokens.

Then, select Custom Token to add the relevant contract address, symbol and decimal precision. Contract addresses (PoS-WETH in this case) can be found on this link:

You will need to add the child token address to view balances on Matic mainnet.

The decimal of precision is 18 for WETH (generally, for most tokens decimal of precision is 18).

Once you fill in all the fields, you can click on next and your token will be added.