Specifications of Microcontrollers Is it all same?
The list of parameters that you have quoted is just the beginning.  The important things (for me) are the number and type of peripherals built in to the processor silicon, which pins are they available from and how they are driven by the software.  The different processor families, even different ranges within the same family from a single manufacturer, have very different peripheral sets and operating modes.  Providing the level of support that is available through a major operating system, such as you would get on a desktop system is just not possible within the processor memory resources.

All of the peripheral devices have their own set of control registers, all of which will be different to those found on other processor families.  This has to be documented to allow the device to be used.

A simple example.
On the AVR32 most modules have separate registers to set and clear the interrupt enable bits for a peripheral.  To enable an interrupt you write a 1 to the bit in the enable register.  To disable it you write a 1 to the bit in the disable register.  To find out if it is enabled you read the bit state from a third register.
On the MSP430 to enable the interrupt you write a 1 to the single interrupt enable register, to disable it you write a 0 to this bit, to find out whether it is enabled you read the state of the bit in this one register.  

Without all of the detail in the datasheet you would never manage to work this out.  This is then repeated for each of the registers available for every peripheral within the device and you can see why the datasheet gets so big so quickly.

I have used a number of ARM, AVR32 and MSP variants and, while each comes with a number of header files and device libraries to allow the use of the on chip peripherals, these libraries are, to be quite honest, of variable quality (the LINT errors from some of the code have to be seen to be believed) and often do not cover the interrupt mode of operation that is needed for my application.  The information in the datasheet allows me to write and debug the software to get the optimum performance for my application.  My requirements, normally extremely low power dissipation, will mean that my code has to be written using the device in a different way to that where the priority is data throughput.

Your final point about AVR not being used much in professional systems - there are many processors in the market and each has its strengths and weaknesses.  I tend to use MSP430 because of its very low power requirements.  I have used AVR32 in preference to the ARM-M3 for the same reason.  Price is very important.  On chip peripheral mix and their fit to my application will make one processor a killer and another very similar device a dud.  For me the AVR and AVR mega did not have the right mix of processing power, peripherals and power consumption and were discounted very early in the processor selection process.