Who pays for wallet balance updates connecting to the blockchain?

I've been wondering about the cost of reading data from the blockchain. The nodes get paid for adding blocks, confirming transactions, executing smart contracts if appropriate on a specific blockchain.

Let's say I'm changing wallets, which needs to connect to the blockchain to retrieve data and update balances and information. Who pays for this cost of read-only? Same goes for web based block explorers.

I'm having a difficult time appreciating how the administrative work gets done. I can't imagine nodes would want to subject themselves to significant non-direct revenue generating effort.