Yeah that's your issue.
As you work, the paint on the brush dries up and gets thicker, every time you add paint to your brush that thickened paint gets diluted so the brush remains supple. Outdoor that thickening happens faster. So the brush get sticky after a while. Washing your brush at the end of the day acts as a reset for the next day. If you freeze it, then yeah, your paintbrush is still going to be near piece of shit it was the previous evening and will be useless before lunch.
Indoor that process is slow enough it can take several days before the brush get sticky. If you've been freezing it, by the time the brush gets sticky, you've been using it long enough it's not much of a waste to toss it and get a new one.
I'm sure you figured that one out, but I figured that breaking it down might be good for novices painters lurking.