The assumption in the question would seem to be that the two largest parties in a proportion representational parliament would become the two largest parties in a first past the post parliament. While this is quite logical there are many things that might mean this is not the case.
For the top two parties in PR to come out at the top in FPTP they would need a majority of seats this might not reflect well how their votes are currently distributed i.e. 20% across a country doesn't necessarily main 20% in all seats or in one specific seat and 20% in one seat might not be enough to win it anyway
The tendency in FPTP for the voters to choose the top two parties/candidates arises from the perception that other parties aren't likely to win in their seat. This is a position that is likely to take time to set in and while this is happening other parties may maneuver themselves into leading in the most seats.
other points to consider include:
- Candidates elected under one system are not likely to wish to abolish it in case the replacement system leads to them losing their position
- Some countries require either a referendum or larger then usual majority for such major constitutional change. The two larges parties may not be able to produce the majority
- FPTP is/seems/may be perceived to be less democratic and therefore a backwards step
- This is a significant change in organisation terms for political parties and perhaps in bureaucratic terms (depending on how it would be implemented)
- This would be a massive sweeping change requiring lots of explaining to the public lots of parliamentary time and so forth - the two main parties may feel there are more important things to do
- Popularity - if this isn't popular with the electorate this could cause a swing against the parties leading to lost seats
TLDR; They don't do it because it's a lot of effort and the two biggest parties might not win afterwards