The Belgian / Franco border was relatively demilitarized compared to the Franco-German border. It also allowed German troops to flank France and decimate their troops

But to answer the question, they honestly believed that Britain would remain neutral. Britain was a historic enemy of France

Essentially, they hoped the invasion of France through Belgium (the long designed Schlieffen Plan) would be a quick affair. The classic understanding is that all the major powers of Europe realised all other powers would mobilise, and Germany wanted to secure a victory before anyone else could. The problem was, it was kind of like a large, continent wide, game of diplomatic chicken. To act first would have you labelled as the aggressor, but would get your troops on the field first, giving you a huge advantage.

They hoped (assumed) Belgium would let them in (Belgium was quite weak and their defense strategy was, largely, treaty based) and they would occupy France (as /u/jb567 points out, by flanking the French in a surprise attack) before anyone could get their troops in place. They had memories of the very successful Franco-Prussian campaign, 45 years earlier, and were hoping to replicate that success.

There were also diplomatic talks trying to convince Britain to remain neutral, or even side with Germany.

Getting bogged down in Belgium was not part of their intentions, and at the time, optimism won out.