MacOS – MacBook keeps switching between the 2.4Ghz and 5Ghz wifi networks for no reason

Run iStumbler from your Mac. (Get it here). Look at the other wireless networks in your vicinity - and check what channel(s) they're running on. You want your device to be as far away from those channels as possible. There is a 3-channel overlap. That means, a device on channel 1 will interfere (albeit only slightly,) with a device on channel 3 (or 4? I forget.) So if lots of devices are on channel 6 (most come out of the box on this channel,) you want to be on 9/10/11 or 3/2/1. I almost always just use 1 or 11.

There is also a "use network robustness" option which I think increases range but decreases bandwidth. See this Apple Article.

You are suffering from network congestion. Channel 11 is a very common channel and most 2.4GHz routers ship with that channel default. There are simply too many routers using channel 11 around your house.

However, not many people in your neighbourhood are using 5GHz channels, so that is free from network congestion.

The solution to this problem will be to change your 2.4GHz channel from 11 to something else.