I agree. When you click the holdings button on the Investment Register, you can adjust the columns there, so the capability exists. I hate horizontal scrolling. And Quicken employee, please get them to adjust the default width of each column to the contents of the register when you open it. Should be an easy fix.
I'm not a Quicken employee, but in QWin 2018 Premier, when I am viewing the Transaction List (Register) for an Investment account, I can mouse over the little line between most of the columns and click and drag the double arrow to adjust the width. Do you see something different?The Description column has a minimum width. I think this is because when the transaction is selected, multiple fields are shown there.
I also would find it valuable to be able to resize the Description column in the investment register to a smaller size and to customize the order and presence/absence of columns. Meanwhile, to avoid horizontal scrolling, I find I can see all the columns if I go to the View menu and undock the Account Bar. Annoying extra clicks, but much easier than repeated horizontal scrolling.