As a moslem, we know that the answer of "Assalamualaikum" is Wa'alaikumsalam. So, when we pray, and we are being an Imam. In the last raka'at, we say "Assalamualaikum", my question is: "Why the ma'mum do not answer his salam with Waalaikumsalam? Answering moslem salam is fardhu, right?" Thanks.
It seems you are mixing two different matters here:
- "as-Salamu alaikum" as a greeting: Here especially Imam ash-Shafi'i is known to have declared that saying "As-Salamu Alaikum" (to start greeting) is sunnah, but Answering it is fard/farz!
And you can read more about how to replay here (I recommend you to read my answer there).
- And "as-Salamu alaikum" as the end of a prayer. First you should know that prayer is an independent worship so it has it's own rules so we know that taslim (saying as-Salamu alaikum) ends a prayer from this hadith.
And we know that we should pray the way we have seen our Messenger (peace be upon him) used to pray or confirmed his Sahaba (May Allah be pleased with them) in their way to pray:
For example you will find in al Ada-ab al-Mufrad from Imam al-Bukhari:
Abu Sulayman Malik ibn al-Huwayrith said,
"We came to the Prophet, may Allah bless him and grant him peace, being young men of a similar age. We spent twenty nights with him. He thought that we desired our own people and he asked us about those of our family we had left behind, and we told him. He was merciful and kind, and said, 'Go back to your family. Instruct and command them. Pray as you have seen me praying. When it is time for the prayer, then let one of you give the adhan and let the oldest of you lead the payer
on the other hand @azam already quoted in his comment a few things about this taslim. Personally i'm not sure if the matter that we say salam to the Malaika has any reference, but i would be happy to know it for sure. Also except the Hanafi school all sunni school's consider the 1st "as-Salamu alaikum" as fard/farz and any other as sunna and therefore as the end of the prayer and the Maliki therefore regard it as mandoob to answer the taslim of your Neighbor and Imam by saying "as-Salamu alaikum" in their direction!
And Allah knows best!