Well the reason that doesn't make any sense is that's not at all what happened. I'll pull most of this from Arrian's Anabasis of Alexander, books 5 and 6 cover this in more detail.

Following his defeat of Porus, Alexander did a couple of things. One he spoke to Porus, as you mentioned and Arrian indicates that he did think Porus was a very cool guy, notably because he fought pretty much until the last of his troops had routed then rode off on his elephant like a boss.

Alexander sends a messenger to call him back, talks and then basically gives Porus back his kingdom as part of the larger Macedonian Empire. This feels weird to us today where wars tend to be fought over regime change. But Alexander wasn't trying to completely reshuffle the administrative state, he mainly just wanted to be in charge, similar to the old Persian method of leaving local elites in power but putting Persian governors in charge.

Afterwards, Alexander set off on a few different things. He founds 2 cities, one where the battle against Porus was fought and the other at the ford where he crossed the Hydaspes.

From there, he spends some time consolidating his control. He wages a war against Porus's neighbors, the Glauganicians and beats them, takes a bunch more cities crosses another river and pretty much wipes the floor with all comers. At this point generally speaking, he is heading southeast-ish and stops at the river Hyphasis, pretty much intending to conquor the rest of India.

So why did Alexander stop and turn around?

Well, because his whole army said, "hey bro we're tired and want to go home".

At this point the army was facing yet another river crossing, which would no doubt be very dangerous, even unopposed(during the last river crossing a number of men had died due to boats capsizing) Everyone starts grumbling, and Arrian records that even those who were moderate 'bewailed their lot' ie started complaining.

Upon hearing this Alexander calls a meeting with his lieutenants and proceeds to give a speech, basically saying that they were free men and could do whatever they wanted, but then reminded them that he had conquored the world and shared the spoils(is your all rich) along with the dangers(is I've fought in every battle with you). He pretty much calls anyone who wants to go back a coward.

We also get a interesting take on how the ancients viewed geography, Alexander seems to be of the view that they are almost to the edge of the world island, ie the shore of the Eastern Sea that surrounds the world. This is of course, not true at all, but Alexander basically says, hey guys, we're pretty close, I'd like to take over the world. Plus he doesn't want to leave anyone behind to cause problems, which is understandable.

So he gets up and says all this, trying to inspire his men, and when he's done, he says that if anyone disagrees they should speak up. On a personal note, it feels a lot like he doesn't expect anyone will. It's almost like a scene out of a movie where the villan kills someone then asks if anyone else wants to voice an objection.

But it's likely a sign of how upset everyone was that a guy called Coenus stands up and explains that, while he is completely onboard with Alexander's world conquest plans, he knows some other guys who aren't and he's just speaking for them... Wink wink.

His argument boils down to, it's great that we're rich, but I'd like to see my parents and take a break, also a lot of the original guys are dead now, so the money doesn't do them much good. We're tired, so let's go home, you can raise another army of young idiots and go off on round two and we'll all be happy.

Everyone cheers.

Of course Alexander is not very happy about this, he goes and sulks for a couple days hoping everyone will change their minds, but they don't and eventually he agrees to turn home.

The journey home is its own adventure, but this post is getting long, so in short they travel South to the Indian ocean, part of the army sails back to Babylon and the other part marches. Unfortunately Alexander dies in Babylon and the empire falls apart. But as for why Alexander turned back his army refused to continue, so he was going back to rest and gather a new force for another campaign.

Ref: Waldemar Haeckel "The conquest of Alexander the Great" / also discussing David Lonsdale's view

The explanation put forward is that Porus new kingdom, (which was strengthened and enlargened) was intended to be a buffer state at the borders of the empire.

The founding of two cities, Nicea, and Becephalia across the Hydaspes (Jhelum) are said to be typical of alexander's frontier policy.

Arrian and later historians have remarked upon the 'Mutiny on the Hyphases' . [Alexander had given a speech announcing his intention to cross the Hyphases and subdue the Gangetic kingdom of the Nandas; his men had refused to follow him, with Coenus pointing out how few were left of those who followed him originally and their longing to see their family etc. After a few days sulk, Alexander essentially abandons his intent]

Haeckal calls this a monstrous fraud, a stage managed drama.

Alexander didn't attempt to make his proposal palatable to his men. He presented in full details the strength and superior numbers of his enemy, to a bunch of demoralized men. Coenus had mentioned the poor condition of the army and equipment. Alexander didn't mention that new armor for 25000 men was on its way, along with reinforcements,

The effect was to essentially place the blame and the guilt for turning back onto the shoulders of his men.

Porus's kingdom, had no mention of garrison,satrap or strategos, and would have lain across Alexander's supply line back west

Haeckel points out that there are 3 things : What Alexander did, What Alexander wanted the world to believe (the king's propaganda) and later propaganda about the legend of Alexander.

And that historian preconceptions about his intent to push forward into the unknown against substantial forces have blinded them to a much more sensible strategy.

The battle with Porus had given Alexander's army all they wished to handle, and across a deeper and broader river lay a much larger and mightier foe

Whether the mutiny was incited/stage managed or not, Alexander and his army/fleet instead went south down the Indus to the Indian ocean and then back west. On the way south along the Indus, lay several battles, incuding one where Alexander took an arrow through his chest. Although there would be more fighting, there would be no more individual battlefield exploits for Alexander.