Why is it we don't resist those who exhort us to seek personal purity, which is beyond our power; but we do resist those who exhort us to earnestly seek to do good works, which are within our power?

Seeking to be pure and seeking to do good works are both equally important. They both require God's power and anointing, they both take vision and dedication and desire, but they aren't both considered the same by many Evangelicals.   

I think the Grace message has been emphasized so strongly that many think Good Works are incidental. The verse below shows the expectations Paul had for the believers to press ahead in faith and "Their every desire for goodness, and every Deed prompted by faith. 

"...We constantly pray for you, that our God may make you worthy of his calling, 

and that by His power He may bring to fruition your every desire for goodness and your every deed prompted by faith." 2 Thess. 1:11