Because almost nothing under a sink matches.

Sizes differ between makers and markets.

Materials differ between decades.

And nothing lines up on the same axis.

The fixture and the pipe were placed separately.

They get used to add too much length.

Which makes a long horizontal run.

With little or no fall along it.

That holds water and collects debris.

And the trap gets blamed for blocking.

Because every adaptor is a joint.

Made blind, in the worst position in the room.

By somebody lying in a cupboard.

More fittings means more joints.

Use as few as will do the job.

The right washer, seated square.

Not more force on the nut.

Overtightening distorts the washer.

Which is why it then weeps.

Hand tight plus a turn is usually enough.

A last resort rather than a first.

They solve any alignment instantly.

And then they sag in the middle.

Which holds water, like a level run.

Support one along its length if used.