The answer is just rushed designing by the designers. The Panther was rushed into service on top of having multiple modifications and new designs put on the tank before production. While the Tiger, while a bit more reliable also had some of these problems.

The original Panther design called for a 29 ton tank, however Hitler wanted other specifications on the tank such as more armor and a bigger gun. This increased the weight by almost 20 tons. These designs were added but the added weight was not really factored in. So this new tank that supported only 29 tons before now must support 20 more tons that it wasn’t designed for. This put a huge strain on all the tank parts like the transmission or final drive wheels which broke often. The tank itself was rushed onto to the battlefield and while successful in combat, overall it’s performance was lacking as many were taken out of action due mechanical failure. While they would continue to re-design and improve the Panther, it would still suffer the same problems as before. And Germany would continue to try and put newer tanks in and variants of existing tanks, they suffered also from reliability issues and part shortages. The Tiger I would not suffer as much as the Panther as it had a longer development time, but much of their problems stemmed once again from too much weight and lack of parts. The Germans would also create many different variants of the Tiger as well, constantly improving it, but much of the problems just stem from the tank stretching much of the parts to the limit.

You can see the problem already of rushed designs and short trial periods. Compared this to the US who frequently delayed newer tanks to being deployed due to unsatisfied testing. The US had to make sure the tank they send 2000 miles away had to work first time, every time. And while the Germans had shorter ranges on their tanks, they could use trains to bring the tanks up to the front as close as possible. Then deploy them where they could use their abilities to the fullest potential. Time was also a big issue, Germany was losing the war by the time the Panther and Tiger were fielded in more numbers. They needed better armed and up gunned tanks to defend their territory and take out as many enemy tanks as possible as they had no hope of out producing the Allies. While these tanks were deadly when deployed right, if it can’t show up to the battlefield and contest or support the infantry, then it is just a 40-50 ton paper weight.