"Main bath RAHI hu" is nonsensical in Hindi.
Perhaps you've heard some UK English speakers say, "She's sat in the corner," whereas US English speakers would nearly always say "She's sitting in the corner." The nature of "to sit" in Hindi is that it's a completed action, so "Main bath RAHI hu" would make it sound like we were watching a slow motion video of your butt getting closer and closer to the chair :)
In general (though this is vague, I know) the continuous form (raha) is used less in Hindi than -ing in English.
Think of Shakespeare. "Do you thumb your nose at me?! Why, yes, I thumb my nose at you!" In current American English we'd ask if you are thumbING your nose. Hindi tends more towards the Shakespeare concept of verbs!
Your reply here has helped me enormously!
A question though, should I report that "I sit" should be an accepted translation? Some users (like me) are British and might genuinely use this phrasing in everyday speech. Even Americans might sometimes say "I sit", such as in the subjunctive tense "It is necessary that I sit". Our imagine you're reading stage directions for a play. "Dave enters stage right. He sits." So we do use this phrasing. What do you think, should it be accepted?
Please clarify what you intend to mean by "I sit." Is it an action in progress or a completed action? The key to Hindi verbs is to decide if they are incomplete, complete, or potential actions, and then locate the action in the past, present, or future. I know that sounds a little complicated, but in complicated cases like this it is better to figure it out natively (through Hindi grammar) than through translation/comparison.
Your description of the Hindi is quite informative, but in US English, we do say "I sit," "you sit," etc. I sit in the balcony when I go to the opera. She sits in the second row of the classroom. The choir sits in the choir stalls. As always in English, the progressive denotes a single ongoing act, whereas the simple present denotes a general state. Maybe the Hindi does something different (note the use of the simple present).
after extensive googling, it seems that मैं बैठा हूँ is the present perfect usage of the verb, seems the implication is that the action of sitting happened in the past, and thus "I" am in the current seated state, while में बैठ रहा हूँ is the present continuous form, which, like one of the comments described refers to "slowmo sitting"