Why does Alkene and Alkynes starts with Prefix Eth and not Meth.

Its because in alkenes and alkynes there are double and triple bond respectively. So they are only formed between two carbons.
In alkenes : CnH2n
If we put n=1
It is ch2 ; in which double bond is not there
In alkynes : CnH2n-2
If we put n=1
Its CH0 ; which is not possible
That is why meth is not possible in alkenes and alkynes