I bet most (all) chips can do 30MHz.

But a very important aspect is that Atmel specifies 20MHz. And that is a frequency that Atmel claims is safe within a specified voltage range and within a specified temperature range.

Most chips can be run at a higher clock frequency if slightly overvolted. But have you tried running the AVR chip at lowest specified voltage? Have you tried it at highest allowed temperature where the internal temperature from the chip will add more heat than the test conditions Atmel used?

30MHz with the chip is something you can play with on your desktop at home/office, when the chip is powered with a laboratory power supply. Not something you can rely on when the chip is in a vehicle or other environments where you don't have control over the temperature or the amount of noise on the power lines.

The reason lots of hobbyists can overclock their PC machines is that they can - to some extent - control the environment stricter than the general test conditions the chip/motherboard/PC manufacturer may require. Using a 1kg cu HSF or maybe water cooling means you can push a chip further.

But an important aspect is that even if you do control the environment and get a pushed chip to run stable, there's a second thing to remember. Excessive heat or excessive currents, through some critical paths on the chip may significantly affect the ageing process. It is well known that overclocked PC processors may run stable for months or years before suddenly starting to hang. The owner must then step-by-step reduce the voltage/frequency just to maintain stability - sometimes the overclocked chip starts to require lower speeds than the specification says, just to behave. Just because of degrading of the chip.

So it's very important to separate what is ok to do for a hobbyist, and what is ok to do for commercial projects. A hobbyist may break things but will only hurt himself. And the hobbyist is expected to understand that the probability of failure will be higher when pushing things past recommended limits.

If a manufacturer have said that clock speed X will be released in Q2, then a commercial company may decide to work with overclocked chips in Q1 just to be able to test source code and learn the timing implications of running at a higher speed. As long as they don't ship the overclocked product, or assume that they can compute reliability figures from the overclocked product. A manufacturer that has a roadmap with multiple speeds may release the higher speed after having tested a longer time just to make sure. Or they may have binned chips before having enough chips to sell. Or they may have needed to stricted up the manufacturing process to get less variances. Or they may have had to redesign critical parts of the circuit after careful analysis. But a customer doesn't know, and can't rely on such things. Maybe, just maybe, the manufacturer just binned chips and the customer did receive a chip that should have been binned as faster. Maybe.