Because outer colonial planets had virtually no way of receiving military-grade equipment to fend off the enemy, they had to turn to buy old planes from local graveyards. Pictured here is an A-7D Corsair II and it's crew, including a Lakhonsaio volunteer sent to help fight the advancing macro scourage and help to move around logistical equipment.

And it wasn't like these vehicles were completely useless. If it could bomb a moving target from insane altitudes or speeds, and or had space for that to be installed it was much better than nothing. Not to mention certainly the cheaper option. Because of this, some places opted to PREFER these alternatives to the expensive and hard to get equipment, all you needed was the factories to make the plane and the schematics. So it's entirely possible you'd see newly made Harrier Jumpjets with modified pylons and systems fighting alongside new space-age equipment.

Again, as usual with "Fewverse" lore posts I'm always open to questions since sometimes i explain things weirdly online lol.