Seforno: Because poor people commonly look after them for pay.

Bava Metzi'a, 57b: To learn a K'lal u'Perat u'Kelal. Refer to 22:8:2:1.

Yerushalmi Shevu'os, 6:6: We lern via a K'lal u'Perat u"Kelal that whatever is subject to 1. breakage, 2. being taken captive, 3. death, 4. K'nas and 5. Ona'ah (overcharging) is included in the Din of Shomer Sachar; to preclude Karka'os

Ramban: 'It dies naturally, falls off a cliff ¹ or is captured by armed bandits', respectively..

Rashbam and Targum Yonasan: 'It dies naturally, is killed by a wild beast or is captured'.

Mechilta: The Torah inserts the case of Meis (bi'Yedei Shamayim) to teach us that, like Meis, which th Shomer is helpless to prevent, Nishbar and Nishbah too he is only Patur if he could not possibly have prevented it, but Chayav if he could. ²

Rashbam and Targum Yonasan: It means that there are no witnesses to testify. ¹