How does an E6 have 6 service stripes?

Retention control point used to be 20 years

Not sure what the confusion is, I have seen plenty of NCO's retire as E6. Retiring as E6 would give you at least 20 years. Maybe he has an MOS that promotion points are Seven Ninety Never and it takes forever to make rank. Not all MOS's have a lot of E7 slots. Maybe all the E7 slots are full and no one has been promoted in several years. This is nothing to be confused about. Maybe he doesn't want to make E7 and is very happy as an E6. There are lots of reasons and they are not for you to question.

I haven't read all of the comments, so I may be repeating:
I could give a better answer if I could zoom in close enough to see his good conduct medals. As said, retiring an E-6 is very common. However, several overseas deployment stripes give a very small opening to wonder why not higher rank. VERY small.
As I said, look at the good conduct medal. 1 award for every 3 years, or every 1 year during times of War. Look at the number of Bronze/Silver/Gold braided knots on the ribbon. At 18 years, he should have a Silver with 1 knot. However, over the last 18 years, I do not recall how many of those we were Congressionally declared "at war." So, he might have a Silver with 2 knots, or a Gold with 1 knot. If he has a Bronze knot, then you know he got in trouble at least once or twice. Or, you can do the investigating, and math, to exactly which bar and knots he should have.
Either way, he's probably an outstanding Soldier, who was happy to stay in an MOS keeping him locked in a long term full billet.

maybe it's petty but, as a prior enlisted guy, it bothers me just a little that I am not allowed to wear my 6 service stripes for the time I was enlisted. Odd that I am allowed to wear my Air Force and Army GCMs, Army NCO professional development ribbon and Air Force NCO PME graduate ribbon but not my service stripes. I earned those stripes just as much as I earned those ribbons.

There's lots of reasons. He could have gotten out and back in at some point or gone Reserve or Guard for a while, or many other things.

I left active duty and went Reserve to go to college. I put more into college than getting promoted. I was stagnant for a while due to my own priorities. I'm now returning to active as a SSG with a college degree and 5 service stripes but only 9.5 years of active duty time, the rest is Reserve. It slowed my career progression but I'm sure there will be assumptions that I'm a POS or got UCMJ. You can't judge a book by its cover. If you want to know, ask. He could have got UCMJ, he could have just decided he wanted to stay an E-6 because he likes being a squad leader (I did meet someone who never wanted to go above E-6 because he hated the idea of being anything more than a squad leader) or many other reasons.

Not uncommon at all spent a while with sham shield preparing for time as a nco then had TIG requirement then ranked up again or had to wait on schools because he ruffled someone’s feather or feel into ranks and put into not priority position for school when schools became available because got promoted without needing school at that time

Positions for promotion is department of the army driven, you have to build points through education, positions worked, levels earned, there is allotted number of the next level promotions army wide by mos(job title): thousands of staff sergeants are on the list, but only a minimum number is granted annually, depends on billets filled and DA board each year, most 20 year retirees are staff sergeants or sergeant first class, all depends on time and grade, time in service, education, position worked, MOS

Breaks in service, or perhaps reserve or national guard in an MOS with too many senior NCOs making promotion difficult. Maybe even just didn’t pursue promotion aggressively or wasn’t current on NCOES.

There are many reasons why some people promote quicker through the ranks than others, also E-6 isn’t a bad career at all.

I retired out of the Army Guard as an E6 with 20 years and 3 months. I was AGR I had 10 years Weekend Warrior and I had 10 years Active Duty. When I went AGR I had to take an Administrative bust to an E5, then in 6 years I was promoted back to E6 due to MOS and Organizational Structure. The reason why I am putting my 2 cents in is this. I was asked 3 times to take an E7 slot, the first 2 times I didnt consider myself ready to take on the responsibility of the platoon. I felt I didnt have a good enough 1st Sergeant, Platoon SGT and I didnt want to carry any of their leadership skills and ability with me if I took it. There was no one to emulate what a good leader was during the time I was in. Needless to say this. I served proudly irregardless of my rank. I took more satisfaction of enlisting and completing my duties to the best of my ability no matter what rank I was.

Why is that hard to understand, yes a SSG E-6 could have 18 years of service as this SSG displays... He also has 5 or 6 overseas stripes which for 18 years service is not a great deal of overseas time....