Fake emails are sent to random addresses in large numbers in hope of catching someone off-guard.
Most scams fail at the first hurdle because the emails don't match your circumstances at the time. The jump out as dodgy. Even so large sums of money are going west: BT's Website suggests about £11bn in the UK in 2016.
People get caught when by chance a fraudulent email just happens to line up with reality. If already expecting a legitimate call from the Inland Revenue, BT, your Pharmacy, Gas Company, the police or whatever, there's a good chance even a wide-awake security expect might fall for a fake.
A more dangerous possibility is leaked personal information being added to emails containing realistic stuff in the public domain like your Bank's address and telephone number. I've had one or two that were uncomfortably close to the real thing at first glance.
Don't believe any emails asking for bank account details or access to your computer. Double check everything, especially when it's expected!
Dave