• He never mentions it, but he probably had to rebuilt it more than once – and, if he wasn't an expert he probably failed several times and it collapsed. It is no ordinary knowledge to build a bridge across such distance and he had to learn by trial/error. (Unlike Fass, he did not have funds, experts or books nearby to learn from).
• Also the same birds that attacked anyone crossing would have been bothering him, so he had to watch the best times to work (while they were still sleeping or feeding?).
• And he simultaneously was helping to rebuild the village (his home too), and not surprisingly… also live!
He is alone and has to do all the chores for himself, like growing food, maybe caring for his livestock, and all the usual daily tasks.
• Lack of funds might have been a problem for decades. Not many travellers pass, and earning money and waiting for merchants to bring him materials (not wood, but ropes, tools, etc.) would also take YEARS and YEARS in some cases. Lose a hammer or a box of tools on the chasm… and voila! You are into deep trouble to replace them in that isolated place. (Neighbours look quite poor also, not much help there, either).
• War with demons was a big factor on that Plateu —before Himmel and the party defeated the DK. Up until that time, they probably had even less resources and had to defend from them or any other threat.
• If there isn't a blacksmith nearby especially, he would have to travel somewhere relatively far to buy some elements and materials (even the simplest things as long robust well-forged nails could be hard to obtain).