1. The people who are pro these types of things are forgetting the concept of shivisi Hashem lenegdi tomid. Hashem is everywhere and sees you even when you aren’t in public and aren’t in front of men. There are certain ways that Jews may not act in even when in private even in solitude!
2. The way you act influences your essence (Sefer Hachinuch- He was a Rishon)
If you act immodest even in private or not in front of the opposite gender in short order you will be acting immodestly even in public. The things we do, EVERY thing we do effects who we are and what we become and acting immodestly allows the midoh of pritzus to take a hold in our personality.