Why does it say that he built Shechem? Was it not rebuilt after Avimelech destroyed it (Shoftim 9:45)?
Radak: Surely it was not desolate for over 200 years! Also, Yisrael gathered there to make Rechav'am king! Rather, he strengthened the walls and built a royal palace. The same applies to Penu'el. It was not desolate since Gid'on destroyed its tower (Shoftim 8:17). Yaravam strengthened also it, amidst fear lest Rechav'am fight with him. Also Rechav'am built fortified cities (for this reason - Divrei ha'Yamim II, 11:5)!
Malbim: He built Shechem to be his capitol. He did not need to build fortified cities like Rechav'am did (Divrei ha'Yamim II, 11:5), for he relied on the majority of Yisrael with him.
Malbim (Hoshe'a 12:4): The 10 Shevatim tried to emulate Yakov. Yaravam wanted that they have Kadosh cities in place of Yerushalayim. He built Penu'el, where Yakov overcame the angel, and Shechem, where Yakov dwelled and built a Mizbe'ach.
