Because for the vast majority of human warfare, formation fighting has been the best way to do it. Running is also a very good way to mess up a formation - it's why a good cavalry charge doesn't go to the gallop straight off, it saves the speed until it's close enough to the enemy. More can always be said on the matter if anyone would like to expand further on the little quirks of Early Modern warfare; for the meantime, I commend to your attention the appropriate section of the FAQ, specifically the headers 'Why did European armies use Linear Tactics?' and 'Mechanics of Linear Tactics'.

Adding onto that section, a few more posts:

• u/StoryWonker has an overview of the relevant factors;
• u/dandan_noodles looks at the dimensions of combat;
• and on the degree of risk and likelihood of lethality;
• and in conjunction with u/PartyMoses, the relative risks of being in formation;
• PartyMoses also covers the dimensions of combat.