[Grammar] watch your step

[kadioguy]

Why not "steps" (rather than "step") in the following phrases?

I assume: the word "step" below is uncountable and abstract.

Does this thought make sense to you?

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break step
: to stop walking or marching with the same rhythm as another person or group of people

fall into step
: to begin walking or marching with the same rhythm as another person or group of people

in step
1 : with the same rhythm as someone or something

2 : matching or agreeing with someone or something

mind/watch your step

1 : to walk carefully

2 : to speak or behave carefully

out of step
1 : not moving with the same rhythm as someone or something

2 : not matching or agreeing with someone or something

https://learnersdictionary.com/definition/step

 

[teechar]

Why not "steps" (rather than "step") in the following phrases?

Because they are set phrases. Use them as they are. Do not try to take them apart or alter them.

I assume the word "step" below is uncountable and abstract.

Countable or otherwise makes no difference, since as I've already said, they are set phrases.
I would say their use is abstract when you're not literally talking about waking/marching, etc.
For example,
The rebel MPs were warned by the whip that they needed to fall into step with the rest of the party.