Netscape was Clark's brainchild. He had left SGI (his previous startup) and recruited Andreesen, who wrote Mosaic at NCSA / Illinois. Clark brought so much to the table with his experience, connections, vision, etc.
You left out $4M of his own money as the initial investment.
There's that too. Clark recounted years later that he felt short-changed by the VCs over SGI and he wasn't going to let that happen again.