Why are Gundams like that?
Depends on the Gundam continuity.
Universal Century, After War Gundam X, 00 et seq., Iron-Blooded Orphans: the mecha have a fusion reactor that emits a handwavium particle which interferes with electromagnetic waves (unshielded electronics, radar, radio, IR, less so visual frequencies but still enough to blur lasers beyond a few kilometers), so combat has to be conducted in-person, and (according to later-written UC worldbuilding) humanoid body plans are easier to control for zero-G attitude adjustments via limb repositioning.
Mobile Fighter G Gundam: Earth is a boxing ring, so of course you'd want the singular mobile weapon representing your country be as easy to control in martial arts by using a motion-capture suit, which means the mobile weapon needs to be humanoid.
Iron-Blooded Orphans, Witch from Mercury: Brain-computer interfaces are most-easily used to control humanoid mecha.
WfM: Maybe something about precision warfare in a setting where mecha developers live in fragile space habitats? But primarily corporate dick-measuring contests.
AGE: A vault of sealed history is leaking archaeotechnology.
SEED et seq.: Arms race from power-assisted spacesuits.
00 et seq.: One Super-Intelligent Dude decides that humanoid power armor is the future of war.
Wing: One secret military group starts making mobile suits because they think they'll get military advantage or something.
Various SD Gundam: Because they're little guys.
The various Build shows: Because they're toys.
