If they enter wrong address, internet won't connect for them. So they call ISP to know their IP. As simple as that. I didn't see any static ip problem here.
It's
FAR from simple. To begin with, most people don't know what an IP address is at all, it's normal that the average user will just call saying "my internet don't works, help!" without any idea that the root cause is a wrong IP.
So the support staff must walk though all the troubleshoting they do now (check the cable, reset modem and the like), plus check if you have the right IP address set. Plus, they have to guide the user on how to do that (you can't expect the average user to know that, really), so you have to know it for all imaginable OSs out there (Win7, Win10, XP, OS X, various Linux flavours, Android, iOS and some more), the real difficulty in doing this is enormous. For a home network where you're the only user with few devices, it's enough, but for a ISP-wide network, it's simply not feasible. DHCP and dynamic IPs solve that.
There is also the problem that someone will mistake their IP and put a duplicate one, now both computers won't connect, so you now have 2 angry clients instead of one, and the chance of a customer DOSing another. ISPs don't want that.
Another problem dynamic IPs solve is that of mobile computers. Notebooks, tablets and cellphones all have wifi and roam around frequently, connecting to different networks. Imagine that for each wifi you see you have to ask the owner for an IP address for you? Or be asked for them when you bring people home? And teach yourself how to set it? And don't forget that when you get back home, you have to set it back to your own IP. Again, not feasible, it won't escale.
Nowadays wifi networks are passed along by providing a network name and a password. With static IPs only, you also need to give one, but you can't write it down, because it must be unique for each user! You have to calculate one considering the current connected people and make sure don't give out duplicates. A DHCP server does exactly this automatically for you.
In short, there are times when you simply don't care
which IP you have, but only that you have a valid one, and those happen to be the majority of times, hence DHCP was created for this very same reason. It solves a problem humans alone are bad at and gives valid addresses to anyone asking, removing the need of people to attend those details. Most people simply don't care what's their IP, as long as Google opens up.
When there is a real need for a fixed IP, you ask for it, or use the default varying one, that's the simple alternative.
