Opportunity, the rover on Mars, is on its tenth year. Wow! But its solar panels are fixed flat on top of it. As are those of all interplanetary landers which I know of. Why aren't they designed to mechanically turn their photovoltaic cells towards the Sun? And the same question about their communication radio antennas towards Earth. They just seem to sit there and waste geometric possibilities. Why? To keep it simple?
The Opportunity, Spirit (RIP), and Curiosity rovers all have high-gain antennas that point at Earth when in use, using a two-axis gimbal. They are used mostly to receive the command loads every sol from Earth. The rovers also have low-gain UHF antennas, basically like a car antenna but shorter, for communicating with relay orbiters around Mars. That path is used to send back the vast majority of the data to Earth. You can get very high data rates to an orbiter due to the short range, so you don't need to point the UHF antenna at the orbiters, and try to slew them accurately as the orbiter flies from horizon to horizon in about 15 minutes.
As for articulating the solar panels, the motors would be massive, and the motors would have to be heated to operate, taking some of the energy benefit from pointing. It is less mass to add more solar cells than to articulate the panels.
As it turns out, we can point the panels at the Sun, and in fact that is being done as we speak. Opportunity is being driven to terrain with the proper slope and direction to point the panels towards the Sun as it gets closer to the horizon in Winter. This strategy has been used every Winter to keep the rovers alive as the Sun gets lower in the sky.
In a way you've answered your own question, "to keep it simple". Don't forget, simple is also cheaper and less risky.
Having solar panels track the Sun to maximize energy capture makes sense, but someone has to design the system and the system has to be built. This takes time and money and it will increase the weight of the lander.
The other thing is the more complex something is the greater the chance of something going wrong. Space probes and landers cost a lot money to make and put into position. The number of failed probes and landers is well documented. A malfunctioning tracking system for solar panels could render a probe/lander useless or severely limit its functionality.