There are two main reasons why you can not talk to a phone directly and why we need network nodes:
1. In case the phone goes offline.
In a perfect world transactions would be direct from phone to phone, but due to the vulnerabilities with WiFi and 4/5G networks this is not possible.
For instance, if you’re doing a transaction phone to phone and someone’s internet fails this will cause the transaction to fail.
Due to the fact the transaction is going through a node it can still complete if someone’s internet fails.
2. Avoids double-spending.
Double-spending is avoided by finding “truths” on the network nodes, Truths are signed proofs of a transaction, from both sender + receiver.