Why does the Catholic Church require miracles for someone to be canonized? Why does someone have to have a miracle (or two) attributed to them before they can  officially become a Saint? Why isn't clear evidence of a righteous lifestyle enough?
Why does someone have to have a miracle (or two) attributed to them before they can officially become a Saint? [...] 

Understood as asking, 

"Why is there a formal Canonization process within the Church and why are miracles required when 'clear evidence of a righteous [life ought to be sufficient]'?"

The latter part of the question is answered by this article Beatification and Canonization | New Advent:


  The Catholic Church canonizes or beatifies only those whose lives have
  been marked by the exercise of heroic virtue, and only after
  this has been proved by common repute for sanctity and by conclusive
  arguments.


Why a formal process?

The answer is in the formula of canonization such as the one below:


  Therefore, today, in a solemn Mass in St. Peter's Square, before an
  immense multitude of the faithful, We have pronounced the following
  formula: In honor of the Blessed and Undivided Trinity, for the
  uplifting of Catholic faith and the increase of Christian life, by the
  authority of our Lord Jesus Christ and that of the Holy Apostles Peter
  and Paul and our own, after careful deliberation, having called
  frequently upon God's help, and with the advice of many of our brother
  Bishops, We declare and define Blessed N to be a Saint, and We
  inscribe his name in the catalogue of the Saints, ordaining that,
  throughout the universal Church, he be devoutly honored among the
  Saints. In the name of the Father and of the Son and of the Holy
  Spirit.


Since raising someone to the altars honors the Blessed Trinity and will be for the uplifting of the Catholic faith and increase of the Christian life, the act was preceded by careful deliberation, calling frequently upon God's help, and with the advice of many Bishops.

Why are miracles required?

People may be mistaken about a person's reputation for sanctity but not God. Before the real Marcial Maciel came to light, if one went by the works he founded, one might think he were a living saint.

The article Beatification and Canonization | New Advent quoted above states:


  the Church [sees] in the saints nothing more than friends and servants
  of God whose holy lives have made them worthy of His special love.


After Jesus performs a miracle whereby a man born blind receives sight, the gospels testify that, '[we] know that God does not listen to sinners, but if any one is a worshiper of God and does his will, God listens to him.'

Therefore the miracles wrought through the intercession of the friends and servants of God testify to the now definitive friendship in heaven in God's presence.



Endnotes:


Please note that there are saints in heaven not formally canonized whose Solemnity [All Saints] is celebrated on November 1.
Saint Paul referred to Christians as "saints". This is the calling for everyone.
For canonizations, the pope may, and does, use one of two methods, each constituting a new species of canonization, i.e. formal canonization and equivalent canonization. [cf. the New advent article linked above].




Further reading:


APOSTOLIC CONSTITUTION DIVINUS PERFECTIONIS MAGISTER | Pope St. John Paul II [the Great].
NEW LAWS FOR THE CAUSES OF SAINTS promulgated in 1983 | CONGREGATION FOR THE CAUSES OF SAINTS.


Please note Can. 1403 §1. 


  Special pontifical law1 governs the causes of canonization of the
  servants of God.


1. Please see the preceding bullet points.