The secretive world of write offs

I've been wondering about this for a while. When companies buy one another, they obviously find "synergies" to lay off people and functions they don't need. But the big question I have is Write-Off's.

Traditionally you'd think about a write-off as something you bought but it is not worth as much as you bought it for. Here is where the question comes in. Why doesn't every company when they acquire another company write off a % of it?

I learn using examples if someone could explain:

• Company A: Worth $10B
• Company B: Worth $5B
• Company A buys Company B for $7B

Now comes some of the possible scenario's:

• 2 years later Company A writes off $2B becase they overpaid for what the company was worth
• 2 year later Company A writes off 50% of the company because the combined company isn't working out
• 4 years later Company A writes off all of Company B as being a bad decision

Some questions are:

• Why wouldn't Company A want to figure out how to write off any aspect of the business?
• When a write-off happens is it like stock's where the company can only write off a certain portion of their tax bill (e.g. you can't write off stock loss's for your taxable income of your employer)
• What is needed to prove what they want to write off? Do they just hire one of the big accounting firms to come in and audit and provide the value's?