why does godzilla look different all the time?
Different suits. They would obviously take a lot of abuse during filming so they had to constantly make new suits and they would tweak or modify things every time. By the time the Heisei series rolled around, they had come up with stronger, much more durable suits hence why his look hardly changed from Godzilla vs. Biollante to Godzilla vs. Spacegodzilla (but they still tweaked and modified a few things). As for the Millenium series, most films stood on their own (with the exception of Godzilla X Mechagodzilla and Godzilla: Tokyo SOS).
Some of the suits from the Showa era are the same though:
Mothra vs. Godzilla/Ghidorah TTHM: Same suit...slightly modified
Invasion of Astro Monster/Godzilla vs. The Sea Monster: Same Suit
Destroy All Monsters/Godzilla's Revenge/Godzilla vs. Hedorah/Godzilla vs. Gigan: Same suit
Godzilla vs. Megalon/Godzilla vs. Mechagodzilla/Terror of Mechagodzilla: Same suit...slightly modified