Why do the highest temperature levels occur at midday?

The highest temperature of the day typically occurs well before sunset but well after local noon (which in summertime is about 1:00 PM in areas that use daylight savings time). The reason for the lag between noon and the time at which temperature reaches a maximum is because is that the Sun continues to warm the ground, water, and air well after noon has passed. It's only when the Sun gets low enough on the horizon that radiational cooling to space exceeds the diminishing radiational heating from the Sun.

There's a similar four to six week lag between the longest day of the year and the hottest days of the year. Just as the Sun continues to provide warmth during the day well after local noon, the Sun continues to provide warmth during the year well after the summer solstice.