Why do shaykhs and religious scholars begin their speeches with "All praise is due to Allah, and peace and blessings be upon the Messenger of Allah," and is it permissible for an ordinary Muslim to do the same?

The Sunnah is to begin sermons and speech with praise to Allah and prayers upon the Prophet (peace be upon him), based on his (peace be upon him) saying: "Every speech that does not begin with praise to Allah is amputated (incomplete)," and due to his (peace be upon him) practice in the Khutbat al-Hajah (Sermon of Need), and his saying: "Indeed, all praise is due to Allah; we praise Him, seek His help, and ask for His forgiveness," and due to the encouragement to send prayers upon him (peace be upon him).

Based on this, beginning speech with "Praise be to Allah, and peace and blessings be upon the Messenger of Allah" is not unusual, and its doer will be rewarded for it.