you can’t just write off anything you want. You only get to write off certain things, but at the end of the day, a tax write off is just a tax deduction for how much you need to pay, in the same way any normal person paying their taxes does. Just like with personal taxes, you can just reduce your tax liability down to 0 if you get enough deductions.
Corporations obviously work differently than for a normal person, but the same basic principle applies.
Edit: i suppose i should clarify - You can take deductions for investment losses. Normal people can even do this. What you’re referring to would be a deduction along those lines, where you’re “writing off” a loss on your taxes. If you invest $100 in stock, and sell when the value is $50, you took a $50 loss, and can deduct those loses from your tax burden, because you’re required to pay taxes on 50 less dollars that year.