- for insurance reasons it's not allowed for them to let you one once they "departed" from the curb, even if just by 20cm. They can literally lose their job over it and while it is a very exhausting job it doesn't pay that badly. (This wasn't always this way).
- due to how Berlin has historically grown and the congestion issues that brought with it the buses have a hard time being on time(1) on the 6,589 bus stops across 152 (day) lines (~40-45 stops per line). So they try to avoid any additional delays, especially if this delay might cause them to miss a green light.
(1): Berlins love to blame the public transportation provider(s) for this delays. And in some cases (S-Bahn) this is partially true, but for the busses it is much more on the "actual hard problem" side then for S-Bahn. And some of the other problems also go back to politics, like so often.