The site good for about 2-3 years then suddenly the site slows to a crawl and the hosts say the website needs optimization or I need a bigger hosting account.

1 The site is Wordpress. It is updated.

2 Old and no longer supported plugins are deleted and replaced when needed. Sometimes Wordpress gives us built in abilities that plugins used to do and we no longer need them so we delete them.

3 Our traffic remains steady according to Jetpack.

4 We have about 3000 word based articles with two images each which are optimized to be small. None of that has suddenly changed.

5 When we move our website to another host then it is fixed for about 2-3 years.

So why, Reddit folks, is this a continuous cycle?

Does something build up that needs to be purged?
Is this a Wordpress issue in general?
Is there something I can check on Wordpress or set or test that could provide an answer?

Edit: Thanks everyone for the answers and ideas. I have some solid plans forward now. Appreciate it.

Also want to say that social media is often unhelpful and aggressive so I was happily surprised by the amount of genuine help. I have heard that the Wordpress community is a good one and am happy to have personally experienced it.

Big high fives to you all. May your weekend be filled with joy.