If rheumatoid arthritis symptoms improve during pregnancy, why women are not treated with hormones?
Women are actually treated with hormones, just ones that aren't estrogen/progesterone/ect..
People with severe RA are treated with glucocorticoids in order to decrease T and B cell proliferation that further damage to the synovial joints. These hormones have long term health impacts (such as Cushing's disease) and should be used as a last ditch effort.
Hormones such as progesterone and estrogen, which are at elevated levels during pregnancy, may decrease T cell and B cell proliferation to an extent, but they are not as efficacious as glucocorticoids. Furthermore, estrogen and progesterone are related to many other autoimmune diseases: http://www.medscape.com/viewarticle/775536_5
Here is information regarding estrogen's role in decreasing T cell proliferation: https://www.ncbi.nlm.nih.gov/pubmed/8955504
*Note: women who are young enough to conceive without medical assistance may still have a thymus. The reduction in thymus activity by estrogen may decrease RA symptoms. However, older women do not have a thymus and will not experience any effects like this from estrogen.