so if we take a look at full casters multiclassing into another full caster they get normal spell slot progression but if a martial goes 5 levels into another martial they cant get the extra attack? why? giving more spell slots to a full caster who can still upcast (already gave my opinion on upcasting) still gets the higher level slots which are powerful but extra attack is were the draw the line? 

Keep in mind that while the spell slot progression is (sort of) combined between two full caster classes on the same character, they still don't have access to the higher-level spells that they would have if they focused only on a single class.

that's true but its giving them class features spells off another list and more spell slots were as a martial going into another martial is getting less then what a full caster gets

Martials tend to be more front loaded- the biggest buff from Rage is the damage reduction, which always scales. Monks get a d6 Unarmed Attack w/ ability mod to damage as a Bonus Action, and now both Monks and Rogues give Bonus Action Dash or Disengage. Fighters have Second Wind for bonus action healing and Action Surge for doubling your attacks for a round, both of which refresh a use on Short Rest.

even front loaded its not much compared to the power of spellcasting 

Quote from UnkKnellnapper >>

so if we take a look at full casters multiclassing into another full caster they get normal spell slot progression but if a martial goes 5 levels into another martial they cant get the extra attack? why? giving more spell slots to a full caster who can still upcast (already gave my opinion on upcasting) still gets the higher level slots which are powerful but extra attack is were the draw the line? 

This isn't even comparing apples and oranges, it's comparing apples and orangutans. Stacking extra attack wouldn't even be the best reason to multiclass between martials if you could do it

If you have ever seen a Barb fighter utterly Mollywallop something with a reckless attack on the second half of an action surge only to apply a topple to them so EVERYONE and turn them to paste, then you would realize that the multi Martials are not in fact hold the short end of any sticks when it comes to damage output.

With weapon maneuvers and great output, spells are no longer the end all be all.

thats great in combat and honestly damage isnt even the full problem its everything else spells can do that martials cant 

Except they're all running on a single resource, so anything they do out of combat is one less thing they can do in combat. Also, you're significantly short-selling the boosts classes like Barb and Fighter got in 2024- a 3rd level Barb can Rage for STR on checks like Stealth and Survival, Fighters can spend Second Wind to boost a failed skill check and get the charge back if they still fail, etc. Theoretically casters have this vast and powerful repertoire, but in practice it's reliant on having the right spell on hand- which only half of them can try to adapt on a day to day basis rather than having a fixed list- and even once you get into tier 3 the resource pool is limited enough that it's a trade off to get major utility on an active day.

Quote from UnkKnellnapper >>

thats great in combat and honestly damage isnt even the full problem its everything else spells can do that martials cant 

 

 
you specifically were talking about extra attack and the like in your original post. If you meant more than combat then you should have said so. Also you have moved the goalpost to just spellcasting in general and not your original point of contention, Multiclassing spellcasters Vs Multiclassing Martials, using only language that indicated combat focus.

You are now having a whole new conversation, so of course my answer to the original won't fit. 
You also seem to be forgetting that Barbarian can now use their rage charges to get advantage on social checks as well. You just need to put a little thought into how to use Martial classes in roleplay. Maybe you know optimal guard rotations, maybe you know where things are generally stored in barracks or how to set up look outs.
Use your imagination, and you can find things that a Martial would be be good at that don't require spells. An experienced fighter doesn't need to know how to cast divination in order to guess certain courses of action are tactically unsound.