They're running out of everything. They can't even find shitty cars to steal from the local population anymore. They've all been destroyed.
So they have to use horses. Like World War I. Horses? Outdated? You don't say... It's all that's left. The infantry travels on foot now near the front line due to no other option, so they pulled horses out of the APC and IFV shed.
It's cool, they can name their horse BTR-80.