Why so many post asking to contact via WhatsApp?

[#1]

I have had little success finding a niche that is steady or has multiple available jobs posted so I explore all kinds of job categories but what I noticed the most is even after submitting proposal employers and to chat via WhatsApp at that point I'm out

[#2]

Upwork is filled with scams at the moment.

In any case, you must not contact potential clients outside of Upwork (before a contract is in place) because it can lead to an account suspension.

I'd strongly suggest you go through this post - https://community.upwork.com/t5/Community-Blog/Top-Red-Flags-for-Scams-From-Community-Member-Wes-C/b... 

[#3]

Once hired on a contract you are able to communicate with a client however you would like.  But, if you are going off-system make sure it's something like Slack where you can easily track/save messages or screenshots from just in case.

The business to customer/user interactions are much more fluid on WhatsApp.  It's extremely simple to use and one of the widest growing apps.  That being said, I don't ever use it.  The only one I will use besides slack or email is Telegram since it's big in the crypto space.

[#4]

Because they want to steal your money.