Are there laws preventing the networks from buying these affiliates? It seems odd that none of them have done an ITV and gobbled up a lot of them.
Apparently yes. There's a "Reach rule" that, for example, prevents Channel Nine from reaching more than 75% of all audiences across Australia. So the metropolitan networks must make affiliation deals with regional networks.
(I'm also hearing that the Australian Government is preparing to scrap that rule.)
There have been some cases of the metropolitan networks buying out regional stations - Nine own and operate NDT, the Nine 'affiliate' in Darwin, and Seven own 'Seven Queensland', serving much of the non-remote areas of the state (outside Brisbane).
It's been much more common for the regional stations to buy each other, so the majority of regional stations are now owned by WIN, SCA (Southern Cross), and Prime. That's where we can compare the situation to what happened within the ITV network. Where many local townships once had a TV channel with its own on-air look and, certainly in the early days, it's own schedule, many now look identical, and are known on-air by the parent company's name, eg. WIN, Prime. Scheduling may still differ between states, depending on affiliation particulars, and WIN actually produces 16 different half-hour local news programmes, which will continue despite the Ten affiliation in most markets.
Affiliation is something of a minefield, particularly in areas without full aggregation. Although I believe almost all markets now have three commercial channels, some still have stations with dual-affiliation - even if there are three stations, harking back to the days before DSO. For example, Tasmania had Southern Cross, which had both Seven and Ten affiliation, and (I think) WIN, a Nine affiliate. There was a station called TDT, on digital only, which carried Ten programming - but because only digital viewers could see it, SCA continued with both Seven and Ten affiliation.
I can't quite get my head around whether all areas now have three commercial channels - there are definitely stations that have dual affiliations, but, like the Tassie example, may have three channels. One station, it may be the Nine affiliate in Spencer Gulf/Broken Hill, carries Nine's schedule apart from news and current affairs which it takes from Seven, such as Sunrise at breakfast time rather than Today.
I could go on, but I'll just confuse us all, including myself! Have a look at this wiki page which is pretty much up to date:
https://en.m.wikipedia.org/wiki/Regional_television_in_Australia