Here's why:

* Neglecting Air Resistance: In a perfect vacuum, with no air resistance, the horizontal velocity of a projectile launched into space would indeed be constant. This is due to Newton's first law of motion - an object in motion stays in motion at a constant velocity unless acted upon by a net force. In the absence of air resistance, there's no horizontal force acting on the projectile to change its speed.

* The Reality of Space: In the actual space environment, there are very small but non-zero forces that can affect the projectile's horizontal velocity over long distances and timescales. These include:

* Gravity of celestial bodies: While the initial launch velocity might be high enough to escape Earth's gravity, the gravitational pull of the sun, other planets, or even distant galaxies can still exert a slight force on the projectile, affecting its trajectory over long periods.

* Interplanetary dust and gas: While thin, space is not completely empty. Interactions with dust and gas particles, though tiny, can cause minute changes in the projectile's horizontal speed.

* Solar wind: The continuous stream of charged particles emitted by the sun can also exert a force on the projectile, potentially altering its path over time.

In conclusion:

* In an idealized scenario without air resistance, the horizontal velocity of a projectile launched into space would be constant.

* In the real world, due to the very small forces present in space, the horizontal velocity might not be perfectly constant over extremely long distances and timescales.

For most practical purposes, especially for relatively short-duration missions, assuming constant horizontal velocity is a reasonable approximation.