You're not entirely wrong! But timeblocking is actually very helpful! I've faced the problems you face as well, but the solution is not to quit, but to start tweaking how you assign time.

For instance, I used to block an hour for writing an article, but then I realised that it took me more than an hour. So I started blocking out an hour and a half.

And I also assign buffer time--because I know life is not perfect!

Timeblocking is simply a way to feel and get more control of your life!

My advice would be to stick to it, and start making adjustments.