Because the equivalent terms could give the impression that the “soul ” is an immaterial part of man. That could reflect the mistaken view that it is something like a ghost and not man himself.
ذٰلِكَ أَنَّ كَلِمَةَ « نَفْسٍ » فِي لُغَتِهِمْ تُشِيرُ عَادَةً إِلَى جُزْءٍ غَيْرِ مَادِّيٍّ مِنَ ٱلْإِنْسَانِ يَخْرُجُ مِنَ ٱلْجَسَدِ عِنْدَ ٱلْمَوْتِ ،‏ وَلَيْسَتِ ٱلْإِنْسَانَ بِذَاتِهِ .‏