Why so people pronounce picture as pitcher in Canada?

Everyone I know pronounces it like that. Definitely not just a Canadian thing.

This is quite common in the US as well, and maybe even outside of North America, although being North American, I do have less experience with those varieties. While I would say “pic-sher” myself, that /kʃ/ cluster changing to /tʃ/ in many accents is logical because /t͡ʃ/ is a phoneme in English; /kʃ/ isn’t.

what kind of answer would be satisfying to you? i mean, how long is a piece of string?

Because that's how it's said. (Not a Canadian)

It's place of articulation assimilation, in linguistic terms. Assimilation is when sounds take on the characteristics of their neighbouring sounds, and place of articulation refers to the part of the mouth where the noise is made. So the 'k' sound gets assimilated into the 't' that follows. It's kinda like how the prefix ´in-´ (as in ´inaccurate´) shows up as ´im-´ before a p. (As in ´imperfect'.) That said: I'm from western Canada and the k sound is definitely there for me!

Words are pronounced in all sorts of subtly different ways depending on your accent/dialect

I've honestly never heard of it pronounced pitcher and I'm in the US

It isn't something which happens on purpose, over time as you say words, some bits of the word get muffled out and eventually you get words that have missing sounds Same in lots of other languages, right?

People also drop the "t" (pikšr)

English phonology is funny that way. [k] is not often paired with the [t] sound in English. In many accents, picture would be pronounced [pikshr] or like you said, [pitchr]. Some pronounce it [piktr] sure, but English doesn’t often have that many consonants in a row. Especially not stops/plosives.

Hardly just a Canadian thing.

Pic - Cher for this Canadian.

(Native speaker from Ireland) I always pronounce it like pic-sher, but if someone pronounced it as pitcher it'd be so subtle I wouldn't really notice.

Because of a linguistic phenomenon called "Canadian raising." This affects the pronunciation of certain vowels before certain consonants. In the case of "picture," the vowel /ɪ/ is raised to /i/ before the consonant /t/.