Because then you can only land in one direction.

What do you do if the wind blows exactly along the runway so that departing and landing planes have only tailwind then?

You would waste a precious runway by having such a big obstacle on it.

Moreover, an upward ramp might not be what is needed. Look for example at Lukla. Here the mountains force the runway to be used in only one direction, and it is slanted so that departing aircraft will roll downhill, accelerating more thanks to gravity, and arriving planes will have to go uphill, decelerating more for the same reason, and thus both need less runway.

Because then you can only land in one direction.

What do you do if the wind blows exactly along the runway so that departing and landing planes have only tailwind then?

You would waste a precious runway by having such a big obstacle on it.

Moreover, an upward ramp might not be what is needed. Look for example at Lukla (or, as ManuH mentions in the comments, more generally to altiports). Here the mountains force the runway to be used in only one direction, and it is slanted so that departing aircraft will roll downhill, accelerating more thanks to gravity, and arriving planes will have to go uphill, decelerating more for the same reason, and thus both need less runway.