Why do airplanes use MAYDAY when in danger but ships send SOS?
The difference here isn't between ships and aircraft: it's between Morse code and voice.

The SOS signal is only for Morse code. It's short, easy to send, and easy to recognise. But it's not as convenient to say. It doesn't actually mean "save our souls". The letters were chosen just to form the simple Morse pattern, and "save our souls" is a backformation: it was made up by sailors later, partly as a joke, partly as a mnemonic.

"Mayday" is an English-looking spelling of French m'aidez, "help me". Spoken out loud, it's short, easy to send, and easy to recognise. It doesn't have any sounds that some nationalities can't say (such as r, th, or v). It's a good signal to use as voice, but would be much worse as Morse code, because it's too long.

Back in the days when aircraft were equipped with Morse code transmitters, they would have sent SOS as a distress call, just like a ship. And a ship with voice radio would send "Mayday" instead of SOS. Now that Morse code has fallen out of use, SOS is also disused. You only hear it in movie plots where the plucky hero doesn't have a working radio but can somehow improvise a way of signalling Morse code (usually by holding two wires together on a broken radio).