why people are so high expectation about 5g?

You must not be in the UK, Australia, or the US, but that's okay.

Mainly because of the Hype but also because of the wait. People knew 5g would eventually arrive but now it's being Hyped as being faster than whatever your ISP can provide. 

But these phone companies and service providers need to sell this Hype because it will be a long time before 5g can reach these advertised speeds. 

I remember upgrading from a Galaxy s3 to the s7 and I was using more data. So they will not only sell you a new phone but a new and more expensive phone plan.

You'll see the same hype later this year when the new Gaming Consoles are released.

In my location in the South of UK 4G is slower than 3G. I'll not bother with 5G at all!

The providers are touting it as being super fast and reliable. As a result, the consumers will want the service to live up to the hype. I'm personally not too convinced that it will work as well as they make it seem, at least in the beginning.

huge promotional effort. of course, 5g will use several times more data for downloads and thus enrich the cell phone providers -- which is why they're promoting it

I guess they are hoping that it delivers on its promises to make internet traffic run remarkably smoother.