I have seen very little of this at Trad chapels --

Why aren't there more entire extended families at Mass every Sunday? Why don't we see Grandma and Grandpa of the McTraddy clan, their 6 kids, each of their spouses, and their 25 grandchildren at Mass on a given Sunday, and in the parish hall for a quick get-together afterwards?

I know there are several reasons, which I could list here:

1. Baby Boomer tendency to limit family to 3 children, 4 at most
2. Baby Boomer push for college/success as first priority, kids move out to other states
3. Destruction of America economically in past decades (manufacturing moving to China), to the point that many cities (Midwest for example) are no longer economically prudent to live in. Many move to better places, and they are smart to do so.
4. Kid(s) fall away from Faith due to World, flesh, devil, being sent to Public School
5. Kid(s) aren't Trad, due to family being raised pagan or Novus Ordo (often lose the Faith thanks to Novus Ordo)
6. Kid(s) haven't left the Faith or Tradition, but fell for the Indult and so they no longer go to Mom and Dad's (truly) Traditional chapel of choice.
7. American Individualism and tendency to have no roots; move all over the country in search of $10K salary increase, at the drop of a hat
8. Economic necessity: some areas collapse economically, with little to no job prospects. Also political/cultural/demographic changes for the worse; Detroit, Portland, or San Francisco for example.
9. Modern world difficulties for Trads trying to get married; many end up marrying "less than ideal" either requiring an out-of-state move and/or marrying a non-Trad
10. Mom and/or Dad is a convert, so 1/2 the family is "out" right off the bat

But when I was a kid, and we went to Mass on Sunday, we would see my Grandma & Grandpa on the Mc (Dad's) side, an occasional Mc aunt or uncle, one of my mom's brothers and his family, and one of my cousins about my age on the Mc side. The coordinator and main altar server was Thomas A. Nelson, the little brother of the aforementioned Grandma who attended Mass there. My mom's other relatives were Catholic, but didn't go, either because they were out-of-state and/or they weren't Trad, etc. My mom's parents were divorced and remarried. In her later years, my maternal Grandmother started going to an SSPX chapel, as did her youngest son, who also discovered the SSPX (but again, he was out of state, he moved out west).

All my aunts & uncles, both sides, were raised Catholic and/or went to Catholic School in the 60's and 70's.

There is an SSPX chapel in Canada, in Alberta specifically, with approximately 500-600 attendees belonging to the same massive extended family, the original generation having approximately 15-16 siblings, and each of those having at least 10 children, and they are now on their 3rd or 4th generation, I can’t remember which.

Not naming the family, but if you’ve been to an SSPX Mass in Alberta, or at least in Canada, chances are you’ve come across them. 

A lot of trad children choose not to practice the Faith either because they weren't raised properly or because they want to be bad, or both. I think if people aren't brought to Mass except on Sunday when they are obliged to go, don't pray the rosary as a family every single day, don't attend parish devotions, don't have their boys serving on the altar and their girls singing in the choir, then the kids grow up thinking religion is a waste of their time and just fall away. And that's before you even talk about the kids being in a public school, which is the biggest killer of all.

In these times, nothing short of serious piety will get anyone to heaven.

The younger generations (many of whom were cradle-Trads) bought into Benedict's motu lie, and (as was the purpose) many, many of them left Traditionalism for the indult/new mass.  Many parents/aunts/uncles of these young adults said nothing, or only slightly questioned the mass exodus, and some of these adults eventually left for the indult as well.  Most of these young people left due to A) marry a non-Trad, or B) didn't see anything majorly wrong with new mass, or C) Both.

Reminds me of the Old Testament when God would punish the Israelites for marrying the pagans and losing their Faith.  Nothing new under the sun...

Some of us are converts and our families are either Prots or pagans.

This is the top answer. I think that converts make up a sizeable chunk, if not a majority, of the parishioners at the chapel I attend. My brother and I are converts to Catholicism, and we're the first Catholics in the family for several generations, probably going back to the Protestant Deformation if I had to guess.