Firstly there's the fact that a mains powered tool can cost less than a replacement battery. For instance a battery for my reciprocating saw would have been £120. A new mains powered saw was £110.
Then there's the fact that battery tools run out of power just when you don't want them to. So have two or three batteries and run mains for the charger out to where your working ? Give over!
Then there's the fact that battery tools are always underpowered. You pay more than twice the price for less than half the power. Even the bigger battery tools tend to be rated at 300 to 450W, whereas the mains equivalents are usually 1kW+.
And what a difference having adequate power makes! The job is so much easier. Mains powered tools just glide through the work. The battery equivalent would be slowing chugging along, then stopping due to a flat battery.
Why people buy battery tools to use at home I really don't know. Ignorance of the customer plus the vendor's sales hype I guess. Thinking about it, I bet a lot of people who buy a battery drill have never used an electric drill before, so they won't realise how limited their new toy is.