But round one is easier to steal.. just roll them off. Whereas a square or any such shape, would be very inconvenient to move.
[Edit in response to a question]
Some people have attempted to steal manhole covers in order to sell them for scrap metal. China Daily notes that there has also been a problem with taxi drivers removing manhole covers to "steal water and clean their vehicles".
https://www.bbc.com/news/blogs-news-from-elsewhere-52400235
It's a very common thing to steal. Lots of iron. That's why they use concrete-filled ones. That's why they are stamped so a scrapyard can be checked (or alert authorities).
[Edit in response to a question]
Some people have attempted to steal manhole covers in order to sell them for scrap metal. China Daily notes that there has also been a problem with taxi drivers removing manhole covers to "steal water and clean their vehicles". https://www.bbc.com/news/blogs-news-from-elsewhere-52400235