I find it kind of baffling, honestly. He's been a tank longer/more times than he's been a gun, it's frankly a better alt-mode, and the CW version in particular does a solid job of retaining his G1 look in robot mode.
Do people flip out when Optimus is a fire truck on occasion?
I always liked the idea that G1 Megatron's alt-mode was a simple tool of war. The founder and leader of the Decepticons, built by someone (likely an Autobot) who
thought they could direct and control his violence. He was born without any right of self-determination, so he killed his master and claimed that right for himself.
It's also fits with G1 Megatron's end in Transformers the Movie.
https://www.youtube.com/watch?v=hzNsOGt3bHk
Unicron: I have summoned you here for a purpose.
Megatron: Nobody summons Megatron!
Unicron: Then it pleases me to be the first.
*Unicron makes an offer to Megatron*
Megatron: And?
Unicron: And nothing! You belong to me, now.
Megatron: I belong to nobody!
Unicron: Perhaps I misjudged you. Proceed on your way to oblivion.
*Unicron inflicts pain on Megatron*
Megatron: Argh! No, no! I accept your terms! I accept!
Unicron: Excellent.
And that's the end of G1 Megatron. I think it works better with the idea that Megatron began his existence as a slave, belonging to someone, being summoned here and there by someone, and having overcome that slavery a long time ago, rather than Megatron simply being full of himself. Megatron effectively died as soon as he accepted that he could belong to somebody.
And then, the limiter that was once placed on Megatron (his alt-mode being a tool) is removed for his second life as Galvatron. Galvatron is a gun who can aim himself, because Unicron's preferred leash is that he can inflict pain on command. And once Unicron's out of the picture, Galvatron without a leash becomes that much more dangerous.