Why the word "divide" in "six divided by three" is past tense?

because this verb is used as an adjective
A verbal adjective is a phrase that acts as an adjective by describing a noun. A verbal adjective, in specific, describes the noun by what the noun does. For example, take the sentence "The elephant sleeps." We can rewrite this as the phrase, "the sleeping elephant". Here, "standing" is a verbal adjective describing the elephant. Taking the sentence "The lion fell", the corresponding phrase would be "the fallen lion", where "fallen" is the verbal adjective.

This is a good question. "Divided", "multiplied", and "added" all look like verbs in the past tense. Divided can also be adjective and has an entry in the dictionary as such, but not the other two. Is divided in "6 divided by 3" an adjective or the past tense of divide? If an adjective, what is -ed here doing as far as morphology is concerned? Does it indicate the past as it surely does in "multiplied" and "added" or does it indicate an adjective?
I think it should be taken as the past participle having similar weight as eaten in " an apple eaten...". The adjective "divided" ( a house divided....) should be treated as a functional shift although dividedly and dividedness are derivatives.

To use the present tense, you'd say "divide six by three", so "six divided by three" just indicates the completed action, which is why it's in the past tense. The ultimate result of the division is the answer in the equation.

When you divide six by three, the resultant is six divided by three. Hence the past tense.