The straight-line handstand is important, and there is something you should work towards. Do not however, let it define your practice.
The line is only one portion of the full skill, and it is quite possible to achieve a very high level without having a straight line.

There are two important factors to consider when trying to straighten out your hand stand.

First the physical limitations, in this case shoulder flexibility will be A big reason as to why many people struggle to straighten out.

next factor is a bit more subtle. This is the coordination, sensation, and application of the cues. Someone might be flexible enough, and might be able to demonstrate a nice line with the help of the floor or the wall, but applying the same sensations to the handstand when you are inverted and carrying your full weight on your hands is a different story(not to mention Balance).
This is where you have to deal with managing and determining the proper sensations to feel within your body when practicing the skill, and this is where proper coaching can also make a really big difference.

so remember a lot of times the reason people have trouble getting in a straight line is not because they lack the physical prerequisites, but because they still need to develop the technical awareness.

This was actually my story. I had naturally flexible shoulders, but it took me a few years to get a straight handstand? Why? Technique.

I initially learned to do a banana handstand because my early inspirations were the old time strongman, and that’s how handstands were performed back in the day. It wasn’t until I got into more modern gymnastics training that I gradually started to correct my shape. But I had to modify the technique, and everything felt different than what I was used to and had been doing for years.

The moral of the story? Straight lines are good, but Take your time and don’t worry too much.