Aristotle certainly had never been anywhere near Gibraltar himself. The passage where he makes this claim, Meteor. 254a, is in a discussion of "flow" in the various seas of the world as known to him, and what he says makes it crystal-clear that he has no conception of ebb and flow in the Mediterranean being caused by tides in the Atlantic. And he was writing in a time before Greek scholars really got busy with correlating information from organised surveys. So ignorance is a large part of the answer.

One possibility is that he was reading Plato's discussion of the fall of Atlantis in Timaios 25d and taken it too literally: Plato's story refers to how (tr. Waterfield)

the island of Atlantis likewise sank beneath the sea and vanished. That is why the sea there cannot now be navigated or explored; the mud which the island left behind as it settled lies a little below the surface and gets in the way.

...which is more obviously ill-informed than Aristotle's discussion in Met. 254a. Alternatively it may be that this snippet (about the Atlantic being unnavigable because of mud) was a bit of misinformation that was floating around and known to both Plato and Aristotle, but which no one ever got around to questioning because it was unusual for people to travel that far.

Here's the actual passage where Aristotle makes the claim; it may help show just how poorly-informed he was (tr. Lee):

The whole Mediterranean flows according to the depth of the sea-bed and the volume of the rivers. For Lake Maeotis [the Sea of Azov] flows into the Pontus [the Black Sea] and this into the Aegean. ...For the sea seems to get deeper and deeper, the Pontus being deeper than Lake Maeotis, the Aegean deeper than the Pontus, and the Sicilian sea deeper than the Aegean, whil the Sardinian and Tyrrhenian are the deepest of all. The water outside the pillars of Heracles is shallow because of the mud but calm because the sea lies in a hollow.

So his idea is that the seas get deeper and deeper as you get closer to the western Mediterranean, and this explains the currents in the passages linking those seas. But notice when he says that the Atlantic is "calm because the sea lies in a hollow" (he probably means that the Mediterranean lies in a hollow, not the Atlantic: cf. Problemata 940b.27-9): he imagines the ebb and flow in sea level as caused by the progressive deepening of the sea towards the western Mediterranean, so to fit in with that, he also wants to explain why there's no ebb and flow in the Atlantic(!!!). That obvious misinformation should help indicate how poorly-informed he is about the "mud".

One possible reason is that the Phoenicians and Carthaginians jealously guarded the secrets about their trade routes, especially from their Greek rivals. They even went so far as to insinuate that there were nothing but sea monsters beyond the Pillars to try and discourage the Greeks. Moreover, one Carthaginian captain claimed that he was forced to abandon his ship after he saw it was being followed by a Greek ship while on its way to trade in Britain. The Carthaginian senate reimbursed the captain.

What colonies did the Phoenicians have outside of Gibraltar?