Yeah, it might be impractical for some or all of the reasons, but I see some wiggle room.

Some dickhead accidentally digging through the buried cables is no more likely than the existing dickheads driving into the pylons with their tractors or clipping the cables with raised booms etc.
Happens so frequently in regional areas it's barely even reported on, but there's still plenty of news to find: https://www.google.com/search?q=power+lines+brought+down+by+tractor.

Doing some quick googling it looks like there have been some studies. https://www.nationalgrid.com/sites/default/files/documents/39111-Undergrounding_high_voltage_electricity_transmission_lines_The_technical_issues_INT.pdf

https://onlinelibrary.wiley.com/doi/abs/10.1002/047134608X.W6146.pub2

https://electrical-engineering-portal.com/download-center/books-and-guides/power-substations/underground-power-transmission-lines

Looks like I have some more reading to do. ;-)

I'll add that in theory it takes longer to install underground transmission lines (although community opposition of overhead lines could change that calculus). This is relevant because a significant number of investors won't invest their money unless they have certainty that the transmission lines will be ready when they are finished building. I'd prefer the government just build the bulk renewables (wind and solar), but for better or worse most governments would prefer to outsource some or most of the work to private investment.

Here's a PDF from TransGrid related to HumeLink, which outlines some of the pros/cons for that project specifically.

EDIT: And the full study.