But can't all that be written on the envelope and not the actual letter?

Well yeah, you could also write them on the back in crayon. However, standard form for a company letter is addressee at the top left (so it shows through the window of standard DL size window envelopes), letter reference underneath that (this is the reference code that you use when replying to the letter, which links to the letter in the company's archives) return address & company letterhead at the top right, company registration at the bottom of the letter (legal requirement in the UK, not sure in the US)

Source: I'm Company Secretary (member of the board responsible for ensuring company meets all laws & regulations) for a mid-sized limited company, and in 10 years at this job I've seen only a handful of companies that don't format their letters this way, most of them one-man operations.

For reference I've found this picture of a letter someone claims to have received from Cadbury's, can't say for sure whether it's legit - but this looks far more like a real corporate letter

I see, then.

great job, baby legs!

*Legal requirement in the UK for Private Limited Companies. Not sure about publicly traded firms.