They don't offer the same level of firepower. A portable drone may be faster employed than a helicopter, large drone, or close support aircraft, but it suffers from lack of payload. It also takes longer to get on target, and has it's own set of vulnerabilities. (The vulnerability argument being that even if it is expendable, a drone disrupted or shot down does not help your unit eliminate the threat it was facing.)

A large-caliber, direct-fire gun provides long range, precise, and instant effects. As The Chieftan explains: https://youtu.be/lI7T650RTT8?t=934 (timecode link provided.)