Robert F. Kennedy Jr. says every time he tries to leave his working-out-in-jeans routine in the past, they keep pulling him back in ... because he feels he's in too deep to change it up now.

The U.S. Secretary of Health and Human Services opened up about his unconventional workout garb during a conversation on "The Bossticks," hosted by Lauryn and Michael Bosstick ... explaining he started working out in jeans out of convenience due to his busy schedule.

Then, on the campaign trail in the run-up to the 2024 election -- before he dropped out and threw his support behind then-candidate Donald Trump -- Kennedy says people snapped pics of his attire ... and it blew up online.

Kennedy jokes he's "in too deep" now ... but the jeans tend to bring attention to his department's messages -- so, it's worth maintaining the attire.

Kennedy isn't the only celeb to deadlift in denim ... with stars like Luke Gaines and Tren Brah also hitting the gym in nontraditional workout clothes.

Bottom line ... workout jeans aren't just a fashion choice -- they're a lifestyle change!