system
Given the following post, respond as if you're commenting on Reddit:
human
[MCU]Why was S.H.I.E.L.D. pouring so much money into new Helicarriers when they should have been looking into creating space based vehicles/weapons platforms?

Given that they know there are some heavy hitters out there in the 'verse, having a first line of defense beyond Earth's orbit would make sense rather than a solely planetary based defense strategy.  And yes, I know that S.H.I.E.L.D. wasn't exactly fully in charge of that project but even that group had a vested interest in creating a space based defense vehicle.
gpt
Spoilers ahead for Captain America: Winter Soldier.

The big reason is that HYDRA was pushing for the Helicarriers to subdue humanity. They had no interest in defending themselves from space at the moment, all they cared about was world domination, and the Helicarriers were the means to this end. Presumably once they have world domination, they would divert their attention to what is out there in the vast cosmos.

Another reason is that there didn't seem to be an immediate threat. Even the faithful to SHIELD, such as Fury worried more about threats on Earth, he was pushing for the Helicarriers just as much as HYDRA was.