The thing your're missing is that the Hohmann Transfer orbit takes time, and both Mars and Earth are moving around the sun. For the Hohmann Transfer orbit to work, the position of Mars at arrival has to be opposite the point of of Earth at Departure.
The following image depicts Earth's and Mars' orbit as circular, rather than elliptical to simplify calculations, and shows a sample Hohmann transfer from Earth to Mars. Distance is in AU. All objects depicted are moving counterclockwise.
A Hohmann Transfer from Earth to Mars takes about 0.708 years with the aforementioned simplifications. In that time, our depicted Mars (orbital period about 1.88 years) will move approximately 135° around its orbit, and the depicted Earth will move about 255°.
As such, in this simplified view of the transfer, at the time of departure, Mars is near Western Quadrature (Rather than Opposition) to Earth, similar to its current position as of late July, 2020.
Image is from Solar System Live, UTC 2020-07-25 12:53:47, with Helocentric Longitude set to -55° to rotate the image similar to the graph above. Image uses the actual orbits, rather than simplified circular ones, and the planets are not to scale.