Is there a reason people dont leave their airgapped pc at home and just use public wifi anywhere to broadcast the pre signed tx?
A lot can happen in the time you created your airgapped transaction to the moment you broadcasted it. The price can go down, which would mean that you paid less than you needed to or the value can increase, meaning you overpaid. This is important if you are paying for some goods or services. If you are sending the coins to yourself or the fiat value doesn't matter, than no need to worry about it. One more thing. There might have been a sharp influx of new transactions paying a much higher fee, making your transaction fall down on the priority list. If a fast confirmation is vital, you might not get it due to the time you spent going to the place that has public WIFI.
An air gapped device is an example of cold storage and it is called cold storage because it doesn't have a any connection to the internet and therefore is safer from hacks and phishing malwares unlike hot wallet which is more vulnerable to such attacks.
Now if you connect your air gapped device to the internet it can't be regarded as air gapped anymore.
That's not what OP meant. He is talking about using a public WIFI network only to broadcast a previously signed transaction using his airgapped setup. No private keys would leak in such a scenario in the same way they wouldn't leak if the PSBT was broadcasted using a private/home network.
That's one of the worst thing that you can do for your Bitcoin wallet's safety.
Broadcasting a PSBT using a public WIFI network doesn't impact the security of your wallet or bitcoins. It affects your privacy, but not security.
Electrum is a very good and handy wallet and it's usage is not banned, nor does it affect your privacy.
Each server that you connect to knows all your bitcoin addresses, your IP, and your balance. That affects your privacy.