Why we treat megaloblastic anemia caused by vitamin B12 deficiency with folates (folic acid)?

We don't. Megaloblastic anemia caused by B12 deficiency is treated with b12. Folate (folic acid) deficiency also causes meg. Anemia. Some with B12 deficiency are also deficient in b12, but if one has low B12 ; isn't deficient in folate (folic acid) there's no need to supplement with it. But it won't hurt to take folate (folic acid);if you're among the 1/3 of people with problems converting Folic Acid to it's active form you should take l-5-mthf.