Why emphasise the EBT when anyone who has achieved Arhathood can teach from direct experience?
Without psychic powers, it's actually quite difficult to determine who is an arahant. Fortunately, the Buddha gave some guidelines in MN47 about how to assess a potential teacher:

MN47:4.1: “Mendicants, a mendicant who is an inquirer, unable to comprehend another’s mind, should scrutinize the Realized One for two things—things that can be seen and heard:
MN47:4.2: ‘Can anything corrupt be seen or heard in the Realized One or not?’ etc...

However, it's much easier to verify the validity of the Early Buddhist Texts (EBTs). Many scholars have compared these ancient scriptures as they were translated and written down in many different countries. The EBTs are remarkably consistent in doctrine. Having faith in the EBTs doesn't require psychic powers.
This principle of faith based on inquiry and evidence is fundamental in Buddhism:

MN47:16.1: When someone’s faith is settled, rooted, and planted in the Realized One in this manner, with these words and phrases, it’s said to be grounded faith that’s based on evidence.

Arahants do teach. Sariputta taught, as did Moggallāna. They had different styles. Sariputta was more into textual analysis and Moggallāna more into psychic powers. And both of their teachings were rooted in the Buddha's own teachings. However, without a Buddha to verify their arahantship, we need to rely on the methods of MN47 to establish a trust in contemporary teachers.
Lastly, the EBTs are continuously translated into contemporary languages and are therefore just as relevant today as 2500 years ago.  Samsara goes on as it does while the wisdom in the EBTs remains directly relevant to this day and beyond.