why don't antibiotics reduce our weight?
First, you present as fact that antibiotics therapy (say AT) does not reduce weight. To show this fact, a study must weigh persons before AT and after. Confounders to this measurement are 1. weight differences because of bladder content (up to around 1 liter urine = 1kg), 2. weight differences because of bowel content (100g-200g), 3. Food weight (up to 2kg and more). High salt foods ​can cause water retention that can easily reduce your urine output one day by 500-1000ml (=g), increasing it with the same amount the other day. So your confounders can add up to +/- 4kg easily. But what you want to measure is a change of 1kg.

This means that you need to account for all these. I don't know any study that did this. Please cite such a study before presenting as fact that there is no weight loss from AT.

Assuming there is no weight loss, there are several hypotheses that can be advanced: from the fact that no antibiotic (even broadband) can kill all bacteria, the remaining bacteria in the gut may simply feed off the highly nutritional content of those bacteria cells that were killed. Note that although we know which bacteria of those that we can cultivate in a lab are susceptible to AT, we don't know at all about those that we can't grow, and that is the majority of bacteria in the gut.