So I actually am curious as to how to reconcile this, so did a bit of digging on Sigil and its portals here;
Sigil - 1d4chan
It looks like the majority of portals require specific keys or are only temporary (sometimes both), meaning they can't be relied upon for free trade on a large scale. It also looks like most permanent portals are directly controlled by factions, who use them for specific purposes (ie not trade). For example, the Doomguard has four permanent portals to the Negative Energy plane in the armory, but they use that specifically to connect Sigil to their strongholds there.
My takeaways from this are that if I am a merchant, traveling through Sigil is very unreliable. I probably don't have access to permanent portals, and if I do it's from inside information from factions that are pretty loathe to share secrets. So trade is possible through Sigil, but only on a small-business scale of merchants tracking down temporary portals to maintain business.
If I'm a bigger trader who relies on larger scale, I can't build a business around this. Although trading via Spelljammer has its own risks, it is risk I can plan around. There are stable routes through the Phlogiston which are always traversable. There are raiders and pirates, but this I can plan around by hiring mercenaries, adventurers and the like.
Spelljammer has a lot of chaos, but there are permanent stable routes from world-to-world as well. So travel via Spelljammer is definitely possible, and can co-habit a world with Sigil and its infinite doors.