We all know how benevolent our position to the Jewish slavery is (Rambam Avadim 1):
"כָּל עֶבֶד עִבְרִי אוֹ אָמָה הָעִבְרִיָּה חַיָּב הָאָדוֹן לְהַשְׁווֹתָן לוֹ בְּמַאֲכָל וּבְמַשְׁקֶה בִּכְסוּת וּבְמָדוֹר"
"The master must treat his Hebrew male and female slaves as his equals as to food, drink, clothing, and shelter, meaning that you should not eat white bread, and the slave black bread; you should not drink old wine, and the new wine; you should not sleep on a feather bed, and he on straw; you should not live in the city, and he in the village; or you in the village, and he in the city..."
This law is more severe than hiring a regular Jewish worker. The Jewish slave does not work on Shabbos also and if he has a family their provision falls on the master.
Who would ever want to buy a Hebrew slave on these conditions?