Why do gay people call their partners "friends"?

Because some people are still in the  closet at home or at work. 

Straight people call their partners "friends", too.  If you're not friends with the person you're in a romantic relationship with, you've got problems and it won't last.  Whenever I've been in love with someone, that person was my best friend.  (However, not every best friend I've had turned out to be someone I fell in love with.)

I refer to my "partner" as my husband.  He is my best friend, but everyone knows we're married.  Frankly, none of the gay couples I know refer to their partners as their "friend".  Some (not many anymore call them "boyfriend/girlfriend", but mostly it's spouse, partner or husband/wife.  Anyway, what difference does it really make?

Hopefully your partner IS your friend.  So it's not like this is some kind of falsehood.

Dealing with neighbors, acquaintances, coworkers, merchants and such, gay people quickly learn that some people can become quite upset if they feel like "you're shoving your sexuality down my throat!".  There's no telling why some people are like this - but they are.  You see it all the time on this site:  Someone proclaiming that they're sick and tired of having (homosexuality, bisexuality, gender identity, whatever) either shoved in their face or rammed down their throat.

Unfortunately, that's not just some Y!A troll thing - a goodly number of people are like that in real life.  If you walk up to them with your partner beside you and you get the "and who's this?" look from them, you can get a completely different reaction if you say "my friend James" or "my buddy Sam" than if you say "my boyfriend Felix".  For a significant number of people, NOW you're shoving your "gayness" down their throat.  And you DO get that involuntary widening of eyes and re-setting of shoulders that tells you you've inadvertently shoved something somewhere.

So, considering I'm under no social obligation to tell you ANYTHING about my personal life, it's far easier for both me and you - the person who doesn't want anything in their throat - for me to just say "friend".  So I do.  Consider it a courtesy.

That's why.  I hope I've satisfied your curiosity.

What should they call them? Even the word "partner" doesn't convey a depth of feeling and sounds more related to business. "Boyfriend" and "girlfriend" come across as too juvenile for adults. "The person I'm dating" gets a little clumsy, but I've never referred to someone I am romantically interested in as a "friend". That is far too casual.

Straight people have "boyfriends" and "girlfriends" too.  What's the issue here?

just another lie they tell themselves ...............................

Not gay but my partner is my best friend.