The knowledge, experience, and proficiency are very important, so Tipitaka Memorizer Ariya is required

Even though the monks, full-time practitioner, still required the Buddhist Meditation Professors, why do lay people think they can understand Tipitaka by themselves and enlighten without teacher's taking care? Are they already have memorized Tipitaka enough to taking care of themselves?

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The knowledge, experience, and proficiency are very important.
People cannot manage their listened to others' perspectives, so they hate to listen to the others' perspectives.
People will know how to understand the others' perspectives by the Buddhist Meditation Professors' advisement, then they can be happy to listen to the others' perspectives.
"People's heart is a deep ocean of secrets." The methods to heal and meditate people's heart are so complicated as well, so Tipitaka Memorizer is required to teach those complicated methods like a doctor who took care of his patient.

The similitude is the chemist who has recited, memorized, and learned about the periodic table and the formulas from the past chemist generation to generation.

If the science students want a science teacher, the professor is required, not just want, for Buddhist students.
In Vinaya-Pitaka Mahakhandhaka, Bhikkhu can't live alone without his teacher. Buddha blames and makes a law that a bhikkhu must lives with a Tipitaka Memorizer every day until he can finish Nissayamuccaka Course. Even though he already finished the course, he still has to live with Tipitaka Memorizer until his ordination time is five years old.
It's legal punishment as "dukaṭa-āpatti" for the new Bhikkhu who sleep without his professor in the same monastery every morning, each new offense per day.
Even bh.Potthila, Tipitaka Memorizer, he still needs Tipitaka Memorizer Ariya to teach him the method to enlighten.
In the path of purification and Atthakathā said it required Ariya teacher to teach the meditation and go to meet him about one time per week. And it is better if that Ariya is Tipitaka Memorizer because Tipitaka Memorizer Ariya has more tips and tricks from Buddha than normal Ariya.
The path of purification Part II—Concentration (Samádhi) CH. III TAKING A MEDITATION SUBJECT page 94 ^(online):

1. Because of the words beginning, “Ánanda, it is owing to my being a good friend to them that living beings subject to birth are freed from birth” (S I 88), it is only the Fully Enlightened One who possesses all the aspects of the good friend. Since that is so, while he is available only a meditation subject taken in the Blessed One’s presence is well taken.
2. But after Buddha final attainment of Nibbána, it is proper to take it from anyone of the eighty great disciples still living.
3. When they are no more available, one who wants to take a particular meditation subject should take it from someone with cankers destroyed, who has, by means of that particular meditation subject, produced the fourfold and fivefold jhána, and has reached the destruction of cankers by augmenting insight that had that jhána as its proximate cause. But how then, does someone with cankers destroyed declare himself thus: “I am one whose cankers are destroyed?” Why not? He declares himself when he knows that his instructions will be carried out. Did not the Elder Assagutta [99] spread out his leather mat in the air and sitting cross-legged on it explain a meditation subject to a bhikkhu who was starting his meditation subject, because he knew that that bhikkhu was one who would carry out his instructions for the meditation subject?
4. So if someone with cankers destroyed is available, that is good. If not, then one should take it from a non-returner, a once-returner, a stream-enterer,
5. an ordinary man who has obtained jhána,
6. one who knows three Piþakas, one who knows two Piþakas, one who knows one Piþaka, in descending order [according as available].
7. If not even one who knows one Piþaka is available, then it should be taken from one who is familiar with one Collection together with its commentary and one who is himself conscientious. For a teacher such as this, who knows the texts, guards the heritage, and protects the tradition, will follow the teachers’ opinion rather than his own. Hence the Ancient Elders said three times, “One who is conscientious will guard it.”
   Now, those beginning with one whose cankers are destroyed, mentioned above, will describe only the path they have themselves reached. But with a learned man (Tipitaka Memorizer), his instructions and his answers to questions are purified by his having approached such and such teachers, and so he will explain a meditation subject showing a broad track, like a big elephant going through a stretch of jungle, and he will select suttas and reasons from here and there, adding [explanations of] what is suitable and unsuitable. So a meditation subject should be taken by approaching the good friend such as this, the giver of a meditation subject, and by doing all the duties to him.
   If he is available in the same monastery, it is good. If not, one should go to where he lives.