People who have good handwriting or a poor handwriting mostly undergo the same education. We do not find any reason why people have different writings.
Writing in any form with the hand or typing is a form of communication. Handwriting is the most complex skill for communication.
A child learns to communicate with gestures, some sounds and syllables then speaks and the last communication skill you learnt was handwriting. Hitting keys for writing is comparatively an easier task. It does not involve lot of brain activity. When we write we do lot of subconscious activity.
Handwriting is a subconscious activity of the mind.
We don't think and make our handwriting good poor legible illegible variable etc. It just happens for most of us.
Handwriting reveals our personality our subconscious behaviour our way of responding to situations etc. Handwriting can be considered as a graph of the activity of the mind. There is a whole field of studying handwriting to know personality. It is called as Graphology.
How frequently do we get our ECG for the heart? If a heart patient says that he is not going for a ECG test so that he will not have a heart problem, we would tell him that even if you don't get a test done you can still have a heart problem or may be you are fit. You won't know unless you take a test. Now when you go for a ECG its not your routine activity but still you want it to have peaks and troughs at the right places.
So now if handwriting is to the mind what ECG is to the heart, then would you not want it to be proper?