Unlike excessive and abundant, which are adjectives, abound is an intransitive verb:
1 : to be present in large numbers or in great quantity : to be prevalent
// a business in which opportunities abound
// errors and inconsistencies abound
2 : to be copiously supplied —used with in or with
// life abounded in mysteries
— Norman Mailer
// institutions abound with evidence of his success
— Johns Hopkins Magazine
It is an action, not a description.
Compare it to some other intransitive verbs as a cue for the grammar:
✔ They coughed loudly.
✘ They are coughed loudly.
✔ She falls down and dies.
✘ She is falls down and dies.
✔ They live in a region where oil abounds.
✘ They live in a region where oil is abounds.
✔ [It is] a city that abounds with art museums and private galleries.
✘ [It is] a city that is abounds with art galleries and private galleries.
Having said that, even intransitive verbs can take auxiliary verbs when other tenses and verb forms are used:
✔ They are coughing loudly.
✔ She is falling down and dying.
✔ They live in a region where oil is abounding.
✔ [It is] a city that is abounding with art museums and private galleries.
Although abound might sound similar to excessive and abundant, it is not actually an adjective like them. Therefore, different rules apply.