why the specified maximum power dissipation of a power transistor at an ambient temperature of 25 celcius is much less than maximum power dissipation at a case temperature of 25 celcius?

 

Please refrain from "hijacking" existing threads with tangent or off-topic questions. You can create new threads for new questions using the "New Thread" button on the upper left of the page.

That's not how it works. The dissipation is rated at 25c and gets derated by some fraction of a watt for each rise in case temp above 25c.

 

How are you going to keep the case at 25 degrees C when the transistor is hot?
The case will also get hot then the chip will be too hot. So you must reduce the power.