Why do solar panels not have focusing mirrors?
The simple answer is that the two devices work in completely different ways.
Solar cookers, as well as so-called 'solar thermal collectors', focus the light of the sun to heat something (a pot in a cooker, some oil or ceramics) and the heat is then transferred somewhere, where it generates electricity, usually by some steam engine. So, the more heat, the better.
Solar panels on the other hand use the photovoltaic effect, which directly converts light into electric energy. Light excites electrons to the conduction band and the current is then transmitted somewhere. Too much heat, however, destroys the materials used, so focusing might be a very bad idea.
Although the other answers are correct, I think they are missing the main point. Ordinarily, solar panels work regardless of their orientation. Their performance may be optimal when oriented directly toward the Sun, but if the Sun shines at 45°, 90°, or when it's cloudy, they still receive photons and still generate electricity.
Focussing mirrors, however, need to be directly exactly toward the Sun to be of use. To use them on a solar photovoltaic array would require the user to constantly rotate the array, requiring a Sun tracker and considerable mechanical overhead. For a cooker, that is used for perhaps half an hour or so, this is no problem. But solar PV arrays should operate all day, while the Earth rotates causing the relative position of the Sun to change.
Finally, sunlight is not scarce. If we need more electricity, it is usually cheaper to add more solar panels, than to go through the full overhead of adding focussing mirrors and infrastructure to rotate our entire array.
See also: https://physics.stackexchange.com/a/78848/6319, https://physics.stackexchange.com/a/78918/6319, https://sustainability.stackexchange.com/a/2788/106.
The concept of using mirrors to focus light on a photovoltaic cell has been considered.
See for example MIRROR REFLECTING COST EFFECTIVE PV SOLAR ENERGY CONCENTRATING SYSTEM
The relative costs of the mirrors versus the cells needs to be considered.
Some light will be lost due to imperfect reflection and geometry.
Also, if the light is too intense the cell could be damaged or saturated.