You have bad laces. I thought this too before I tried different laces. Turns out different tensions and elasticities give different strengths of knots.
For example I have some military boots which came with slightly stretchy laces. They NEVER come undone, ever. They were the first pair that switched me on to this, and since then I have always bought laces with slight stretch to them, and the knots always stay done up.
In contrast when you buy a pair of fashion trainers, the laces in them are usually terrible and come undone several times per day as you have noted.