I have only read the OP and a few others. I think one other reason people can stop at 2 IPs apart from the reasons already mentioned is that at the back of their minds they do not really want to be self-reliant.
The reason is that they want to qualify for part government Age pension and the Concessions Card. In fact many financial advisors advertise that they will help their clients to achieve that level of dependence on government welfare. Intuitively, people may feel that achieving this outcome is smart and optimal result with their time and efforts. Some have remarked that they feel entitled to the welfare as they have been paying tax throughout their working lives.
With 2 IPs, an investor is unlikely to be self-reliant at retirement if that is their major assets other than their house and superannuation. A Treasury source has indicated that in NPV terms, a retired person costs $400k in government Age pension and another $400k in other care, excluding housing. In fact, the average Australia is going to rely on the government as for man the average super amount accummulated is about $100k with less for women. Hence, excluding house, to qualify for part Aged pension a single must not have more than about $0.7 million and for couple about $1 m. Two IPs are not likely to disqualify them on the assets test for the Aged pension.
Unfortunately, this is a common mindset of many people.