Why there are signature and public key pair on blocks when transaction verifying is over

I'm student studying about bitcoin and blockchain. I want to know why all blocks have signature and key pair after the transaction is over.

Why I ask this, I'm studying post quantum cryptography (PQC) and almost PQC have very large key and signature size, but now bitcoin block size is limit to 1MB. So if the hardware performance will be similar to nowadays in future, then I think the block size is critical.

I read the bitcoin post quantum white paper, but there solution is that increase the block size to 16MB.

Is there any reason to store the key and signature? Is there something critical vulnerability?

You can get bonuses upto $100 FREE BONUS when you:
💰 Install these recommended apps:
💲 SocialGood - 100% Crypto Back on Everyday Shopping
💲 xPortal - The DeFi For The Next Billion
💲 CryptoTab Browser - Lightweight, fast, and ready to mine!
💰 Register on these recommended exchanges:
🟡 Binance🟡 Bitfinex🟡 Bitmart🟡 Bittrex🟡 Bitget
🟡 CoinEx🟡 Crypto.com🟡 Gate.io🟡 Huobi🟡 Kucoin.