Why is ancient magic always so much stronger than 'modern' magic?

This will probably seem a needlessly large-scale answer to some, but in my opinion, it ultimately derives from the conditions in which Western Civilization emerged, which was basically a bunch of illiterate Celts and Germans slowly developing high culture on the outskirts of the Roman Empire's corpse. That romantic sense of a vanished and vastly superior ancient civilization that knew things we don't has been with us from the start, and we don't seem to be able to get out from under the enthrallment of the basic idea no matter how many metrics we come to outperform "the ancients" on. The same deep inherited sense of "that" presence drives much of the "alternative history" enthusiasm for stories of enlightened high-tech Ice Age civilizations.

It's a trope, basically.

Reality is, modern wizards would absolutely cream the ancients.

Because Daddy God hadn't confiscated it yet.

There is magick and there is magic. They are different. Magic is slight of hand, illusions, parlor tricks and not very impressive.

Magick, if performed properly is real and of nature and the elements of nature.

Bear in mind the world was Matriarchal a long time before it became patriarchal and ruined.

Planned Obsolescence.

It hasn't been corrupted and diluted through the eons. It remains pure, and thus, potent