Why do we say that we "observe traditions" rather than "following traditions" or some other term?
It comes from 'observance' rather than 'observation'. The etymological roots, according to dictionary.com are:
Word Origin & History
observance early 13c., "act performed in accordance with prescribed usage," esp. a religious or ceremonial one," from O.Fr. observance, from L. observantia "act of keeping customs, attention," from observantem (nom. observans), prp. of observare (see observe). Observance is the attending to and carrying out of a duty or rule. Observation is watching, noticing.
Whereas the religious connotations are not as prevalent as they were in increasingly secular societies, the usage still carries over with reference to commemorations, eg
One of the meanings of 'observe' in Oxford Dictionaries is: 'fulfil or comply with'.
I am not sure if we 'fulfil' but we certainly 'comply with' tradition.
As far as my tradition goes, I believe we are the perpetrators of the usage of "observe", abetted by Bible-observant Christians.
There may be other avenues, but as far as I am aware, this is due to the Hebrew part of the Bible (and some Aramaic in the book of Daniel).
In Hebrew שמר is the root radical for words like
שמר in its root form would mean watch/observe.
The Bible, in Hebrew, is full of references to שמר, but in various forms and inflection. People who read the Bible in Hebrew, or who are very attached to their Strong's Concordance, normally have a obsessive compulsion to be precise about the Hebrew concepts we wish to express in English. Personally, I am following the crowd by using the word observe.
For example, in an important subject such as the Sabbath, we normally want to express its importance in all inflections of שמר.
Shall we say we simply keep the Sabbath? Preserve the Sabbath, fulfill the Sabbath, be aware of her, beware of violations against her? Therefore, we simply say
The Christians might say
You might not be aware, but English vocabulary and usage has had very significant influence from the Bible.
It turns out the verb (to observe), as most of the english verbs is too broad regarding its meaning, so having said that, it is true one could use a few synonyms to construct a similar statement within the context you mentioned...
A simple research about the verb (to observe) returns a specific meaning of "to watch or be present without participating actively" when used as an intransitive verb: See reference here
When used as a transitive verb, (to observe) could also mean:
I believe the verb to observe is kinda stick to that context because one not always actively "follow traditions", meaning that one does not always actively participate or experience traditions, but rather "observes" them...