Modern warlords have large quantities of subjects from generations and generations of consolidation, which itself is a variation of joining instead of looting. Gaining subjects is extremely risky, when you don't already have an army.

Really, modern first-world countries are just the descendants of warlordships that ran out of kingdoms to consolidate with and instead switched to taxation, either relinquishing enough power to their citizens to maintain a stable but effectively symbolic monarchy or overtaxing then losing to a rebellion.