Final answer:
Direct detection of exoplanets is challenging due to their faintness and the overwhelming glare of their host stars. Indirect detection methods, which observe exoplanetary effects on stars, are more successful.
Step-by-step explanation:
We cannot use direct detection for exoplanets primarily because they are extremely faint and are usually shrouded by the intense glare of the stars they orbit.
Earth reflects less than one billionth of the Sun's radiation, making its apparent brightness very weak compared to the luminosity of the Sun. This difficulty is compounded when attempting to image exoplanets that are even further away and dimmer.
Indirect detection methods like the Doppler and transit techniques have proven much more effective as they capitalize on the gravitational effects an exoplanet has on its host star, such as causing wobbles or periodic dimming if the exoplanet transits in front of the star.