You’re only able to choose two options, how is that democracy? I thought democracy was about being able to choose anyone you think is suitable to be a leader, not one of two pre-selected people. At that point, it’s not much different to a one-party system, just with two people rather than only one person.
Short answer is capitalist propaganda. The illusion of freedom is important to maintain control and curb resistance.
Edit: my mom was a school teacher and she taught me a trick for manipulating children that is similar. Instead of telling a kid to do something like" little mark do your homework." You give them two choices both of which will satisfy your agency. Like “little mark would you like to do your math homework first or your English?” Which is super equivalent to would you like a democrat or a Republican? Also the only way one of those candidates can get to be one of the last binary choices is completely dependent on financial backing which almost always comes from the capitalist class.