They aren't implementing e2e by default because that would limit the amount of data they collect and sell to intelligence agencies.

e2e encryption makes it difficult to provide fully cross-platform messaging experience

Why?
I can easily encrypt a message on my x86_64 Windows desktop, send it to my x86_64 Linux laptop, and also my ARM64 Android phone, and be able to decrypt it perfectly.

If I can do it then a development team can too.

Why would it be difficult? Encryption isn't difficult (any more than adding any essential feature would be) and is platform independent. It doesn't matter what device you're using, encryption and decryption are commonly used.

You can easily implement AES 256 in JavaScript. So you can tap the web framework on any device and run your app on just about any platform.

The problem is, They no longer get signal intelligence from it and can no longer monetize it.