> There's likely no technical reason why you couldn't use a pc instead of a phone […]

Any personal computer built in the last five years can do anything a smartphone or tablet can in terms of processing power and connectivity. A smartphone is a computer with hardware that enables it to use cell phone networks and make calls.

My inner cynic strongly suspects that Facebook and other similar corporations really like the control they have on the overall user experience on the two major mobile operating systems; i.e., eyeballs on a smartphone or tablet are worth more than those on a general purpose computing device.

Too much freedom on a personal computer; with browsers that feature all kind of privacy enhancing add-ons such as ad-blockers and tracker-blockers. Much harder to monetize.

You are totally right! That's the obvious answer. On desktop you have too much freedom.

> It's required because your phone is where your messages are stored.

So are you saying that a desktop PC's hard drive can't handle storage of some text messages but a phone can?

Of course, he is not saying that, he is just explaining how whatsapp is actually storing the messages.

No, he's saying that in order for the messages to be available, they have to be stored locally and since most people need access to the messages from their phones, it makes sense to do it this way to ensure that all messages are stored on the phone, instead of users ending up with some messages on their phones and some on their PCs.

Neither does iMessage store messages on servers, and yet I receive iMessages on every device I signed on.

But you can't retrieve historical iMessages on a new device.

I'm not really asking for that feature. If I could send and receive messages on the computer without having to open the smartphone app every five minutes for it to restore the connection, I would be happy already;

"Perhaps they've decided that this is a small and declining market."

For consumers (and facebook is a consumer company) it's all about who owns mobile (and can also compete with facebook). They want desktop to just be enough of a feature to be more appealing than other platforms, but not enough that it detracts from mobile.