On PC, you can’t.
This is a legacy of how Bounties used to work in the long, long ago times. Back then, when you entered a game, there was a bonus Act selected and that’s the town you’d start in. Completing Bounties for that Act’s maps gave you a Bounty cache and a Bonus Bounty cache.
Upon completion of the Bonus Act’s bounties, the bonus moved to the next Act in numerical order. So, you’d do the Bounties in the order that the Bonus moved around the maps. The starting map that had the bonus applied to it would change every hour, e.g. if you started a game at 09:55 it might be Act 1. If you started a game at 10:05 it would have moved on to Act II, and so on. As Act III and IV use the same town, this is why that’s a more prevalent start location, as it accounted for 40% of the starting locations.
Many years ago, Bounties were altered so that there was no longer a rotating Bonus Act, and the new Bounty Caches were the amalgamated contents of the standard Bounty Cache and the Bonus Bounty Cache, i.e. you got just as much loot, but you could do the Bounties in any order you liked.
However, this left the legacy of the start town being whichever town would have been the one with the Bonus Bounties active and, thus, it rotates every hour.