Why do I have to pay transaction fee?

Excuse me, but could you elaborate on why exactly do I have to pay a transaction fee when engaging in cryptocurrency transactions? I understand that networks like Bitcoin and Ethereum require miners to verify and process these transactions, but why is it necessary for me, as the sender, to bear this cost? Is there a specific reason behind this practice, or is it simply a standard industry norm? I'm curious to understand the rationale behind this fee structure and how it contributes to the overall functioning of the cryptocurrency ecosystem.