Why is it so difficult to concentrate one pointedly?

Fixed Focus is the characteristic of the Concentration Link to Awakening.

Ceasing of distraction, disturbance, diversion, agitation, mental instability and wavering is the purpose of the quality of Concentration. Incisive certainty is the manifestation of the Concentration Link to Awakening. This stability enables breakthrough to Understanding! Some concentration is present in all consciousness. Training anchoring of attention on only 1 object condenses this focus.

The proximate cause of Concentration is Happiness!
The resulting effect of Concentration is Knowledge & Vision!

Concentration comes in increasing grades of intensity:
1: Momentary concentration with few seconds of one-pointedness.
2: Preparatory concentration of longer yet still unstable quality.
3: Access concentration approaching the 1st jhana absorption.
4: Absorption concentration with fixed mental one-pointedness.

Fourfold is the blessing of Concentration:
1: Sublime Happiness here & now through the 4 absorptions.
2: Assured Knowledge & True Vision of things as they really are.
3: Awareness & Clear Comprehension of all transient phenomena.
4: Ceasing of all mental fermentation by absence of clinging.

The Buddha once said: What mental fermentations should be overcome by development?
If one by careful & rational attention develops the Concentration Link to Awakening based on seclusion, based on disillusion, based on ceasing, culminating in renouncing relinquishment, then neither can mental fermentation, nor any fever, nor any discontent ever arise in him.
In one whose body is calm and who enjoys a pleasurable happiness the mind becomes concentrated. The Concentration Link to Awakening arises right there. He develops it, and for him repeatedly meditating it goes gradually to the culmination of its development.
MN118 [iii 85]