Can anyone explain why I pay an extremely high price for NBA league pass and I don't get all the games?

The last Miami Heat game against the Charlotte Hornets is exclusive to Prime Video, @kcgilbert4018. The NBA may enter into exclusive rights agreements with certain providers such as Prime Video as they buy the rights to show individual games. It would also impact seasonal sports subscriptions such as NBA League Pass. To check game availability and channel listings, go to Basketball TV Schedule & Channels | Where to Watch Today's Basketball Games. The sports site will indicate the exclusive provider when applicable. John, DIRECTV Community Team

Simple answer: Blackout rules and who holds the rights to each game. (Same answer for  all professional sports)

The seasonal subscription (NBA League Pass, MLB Extra Innings, etc.) is for OUT-OF-MARKET IN-SEASON games only.

If you are in-market, you watch on your regional sports network (RSN). (Available in Choice and up with mandatory RSN fee, or if you are in a lower package the Sports Pack add-on is usually cheaper than raising the package).

Sometimes a game is not just regional, but directly in your viewing area. In which case you watch on the local that has the rights to the game (even if that local is not carried by DirecTV).

If game is OUT-OF-SEASON, than RSN and locals are usually the option as only available to those in-market.

In some cases rights to some games may be sold (such as MLB did that with Apple TV as an example). That is where it starts to get murky as that is not simply are you in-market or not.

DirecTV has absolutely no say on this (nor does any other TV provider). This is how professional sports are set up. They set the rules, TV provider can only provide you what is allowed.