I'm migrating an old Joomla 2.5.8 site to 3.6. I made a new Joomla install and installed the latest K2. Then I went in to the MySQL database, did an export of K2 items and categories then imported into new database. I see everything there, but, when I look in K2 the newest items are missing.

Any ideas why the latest items aren't showing?

Did you import the rest of K2's data, extra fields, users tags etc?
Did you import the Joomla! users as well?

Everything was imported - it just doesn't work. So I removed everything and imported with SP Upgrade. Everything came over as registered access. Ran SQL queries to change access and then everything showed but invisibly. I know it was showing as the pagination was there, but no articles were actually visible.

I do not understand why something so simple, 1. is not included with the core K2, and 2. is so difficult.

You are migrating a site into another CMS version and setup.
There is no point to include this in the core.
In the core we only offer the option to migrate core Joomla! articles.

By the way you only need to move the user's and K2 related tables. I have done this numerous times without any issues and without the use of an extension. You simply to need to move the tables in the new database. There are several tutorials available like this one www.youtube.com/watch?v=VCjJ1kWgBlg

The video was of zero use. I actually know how to import SQL tables and I still couldn't follow it. Getting someone with much better skills than me to do it - it just should not be this hard.

Sorry you feel this way.
If you used SP Upgrade and the upgrade did not go as planned you ask their devs to assist you.