Why is "renege" pronounced with a hard “g” sound?

It’s because it never had an affricate there. We spelled it renegue for some time, and some people still do. But just because we dropped the u for ease in spelling doesn’t mean we would change the sound.

The OED has:

renegue, renege /rɪˈniːg/, /-ˈnɛg/, /-ˈneɪg/, v.

Forms: 6–7, 9 reneague, 7–9 renegue, (6 ri-, 7 -neigue, 9 dial. -nague); 7, 9 reneg, (9 dial. -neeg); 6–7, 9– renege, 6–7 reneage, 9 dial. rena(i)ge, 8– U.S. renig.

Etymology: ad. med.L. reneg-āre, f. re- re- + negāre to deny: cf. renay v.

As you see, it had all kinds of spellings since it was adopted into English, but would never have had the affricated pronunciation of judge, only the stop of fig.