system
Given the following post, respond as if you're commenting on Reddit:
human
[Warhammer] Why does the Imperial Guard field horse mounted units in the 40k Universe? Are horses even an effective military unit vs. Orks and the like?

I would guess that horses are acting as a replacement for machines that the imperium of man finds it difficult/no longer knows how to make but I can't see them being effective in the slightest when fighting stuff like the tyranids, orks, eldar and chaos or to an extent the tau.
gpt
A lot of Imp Guard regiments are raised from places the Administratum has designated as Feral-type or Feudal-type worlds, where mounted combat is an already-known skill. If there are humans and mounts already there, it's just easier to use them then to make new vehicles and ship them across the galaxy.

Plus given the population densities and tech levels involved, we can get humans to make more humans and horses to make more horses much more quickly than the Mechanicus can churn out new vehicles and sanctify them for combat.

Two final points... 

1. Normal horses are not the only mounts used. A Venumex, an Ursir, or an Aethexe can be a tough thing to face in a fight, even if you're an Ork, especially when there's someone with a Melta-lance poking at you from it's back.

2. Even if your mount is a horse, there are things that can be done. Either genetic engineering (see the Krieg Death Rider Horses), or just good old fashioned armour and forcefields.

Even without all of that... Humans mounted on horses have protected the Imperium for as long as it has existed. It would be unthinkable to change such a long-established and traditional mode of combat. Lower your lances and charge the guns. The vile alien cannot harm you.

The Emperor Protects!