There are a few reasons why those exercises in particular are chosen in beginner programs.
- They're foundational. Every beginner needs to build on a foundation of something. These are full body strength exercises which are ideal for building a solid foundation of fitness to move on to more difficult tasks.
- They're simple to perform. Push, pull, squat, and lift are movements our bodies were built to do. Most people are capable of doing these workouts. They may not be performed perfectly or the most optimally, but they don't have to be to be safe and effective. People who are not capable of performing them are usually able to build up to it.
- They're easy to remember. The absolute worst thing you could do for beginners is to bombard them with fifty different exercises to learn how to perform. Instead it's much better to give them four workouts that work every muscle group. It keeps things simple and focused.
- They're intense. As stated earlier, they're full-body exercises. They work every muscle group to some capacity. Even bench press once you learn how to properly use leg drive. They really get your heart pumping, and you can get a really good workout in a relatively short amount of time.
This brings us to your primary question
Why most beginner programs ignore the abs?
Because those workouts all work the abs to some capacity. Squats and deadlifts especially do a really good job at engaging ab muscles to successfully perform the lift, otherwise you'd fall over. This will become increasingly apparent as you increase the weight over time. It's common to get muscle cramps in the abs after a really hard squat session.
Actually because of this, it's common for intermediate and advanced programs to make dedicated ab work completely optional. They just let the main workout handle it.
Now, you don't have to skip abs if you don't want. Doing the same four exercises for months can be exceedingly boring. It can be beneficial from a mental/motivational standpoint to add exercises. You can do a few sets of an ab workout after the primary program if you still feel you have the energy to do it (I would argue that you didn't workout hard enough but digressing...). Just keep in mind that the more work you do, the longer it takes to recover so you'll have to stop if you start failing to complete workouts because your abs hurt.