Why is 后 ‘back' but 后天 'the day after tomorrow'?

Hello 后 means “after” or “ behind”

桌子后 behind the desk

五点后 after 5 o'clock

here后天 = the day after tomorrow (after)

后 does not have the meaning “back”

Regards

1.from a position point of view ,后 means “back”, 前  means “front” . examples: 桌子前，桌子后，教室前，教室后

2.from a time point of view ,后 means “happen after /later ” , 前 means "happen before /earlier . examples :2021年前（2020.2019.2018….)  ,2021年以后（2022,2023,2024…)  

hope this helps

后/前 have two meanings in chinese .One of it refers to times ,just the same  as you said in your question,后天、前天；meanwhile,they also refer to directions, as in 前面/后面. The better way you understand the menaing of words it to put it in the sentences or dialogue.

we can also use this way  to rememb, 

前→⚪→后， the time also like a timetable, runs from 前→⚪today→后。

you can see what happend yesterday(前), but you cannot see or know what will happen in the future（后）.

hope this helps.

前 and 后 are the exact opposite of each other

前 means “before” or “ in front of”

桌子前 in front of the desk

五点前 before 5 o'clock

五天前  5 days ago

前天 the day before yesterday (before)

hope this can help you

There are two meanings of 后, one for position description meaning ‘back’, e.g. 镜子后 at the back of the mirror; one for time description meaning ‘after’, e.g. 下班后after work. 前 is the same correspondingly.