Why can't you get married by church again after you've been married by church in the past?

You mean after a divorce? Because Im pretty sure you can if your spouse died. I dont know if it varies depending on the church but I know that you cant remarry in a Cathollic church following a divorce because the church doesnt recognise divorce. Therefore, to the church you are still married and cannot marry someone else.

Because the church does not accept your divorce while the state does accept a legal divorce.

In church terms, you are still married - before god - to the first partner. You just can’t divorce. And you are a sinner for having a new husband. And you can only get married again by church if he is dead and you are thus a widow.

Not true in most cases. Only certain churches may frown on it but in general, people re-marry in church all the time.

cause the church doesnt believe in divorces

You need to get the marriage annulled - then you are free to marry in the church again.

If you mean “the” church; as in the Roman Catholic Church than they don’t really believe in divorce. Annulments are possible in certain circumstances like if the marriage was never consummated, if one spouse knew they were infertile before the marriage but never disclosed that fact to their fiance beforehand, if one spouse became Catholic but the other refused, if they discover they are too closely related, if one spouse was coerced into marriage or it can be proven that one spouse married for reasons other than to have a life-long bond (a con man or perhaps someone marrying only to acquire property or citizenship). Unlike divorce which dissolves the marriage contract annulment means that the marriage retroactively was never valid. Protestant churches allow divorce though. The Anglican church was formed because of the subject. The Roman Catholic Church refused to grant King Henry VIII an annulment of his marriage to Catharine of Aragon who did not produce a male heir. The Anglican church split from Rome and allowed the king to annul his marriage and marry Anne Boleyn who also never produced a male heir but this marriage was ended by her being beheaded rather than divorce or annulment..