You, sir, are an argumentative idiot.
The reasoning stands... Microsoft, Mac, Linux, whatever, operating systems to not "spontaneously bloat-up and slow-down"... there is an impetus, a root-cause... the USER! Let's dismantle your silly argument line-by-line:
"The more you use your OS- the slower it gets." False... this may be the case when you use your computer, but it's not the case when I use my computer... hence, again, the problem is YOU.
"Browser histories get bigger"... well let's see... might that be because you allow it to? Don't you clear your history? Haven't you gone into gpedit.msc to modify the Group Policy settings, disabling the browser's history? User negligence does not equal self-bloat (aka spontaneous bloating).
"and take longer to open"... bloat is not the same thing as disk-sector fragmentation. Seek times will naturally degrade in minute quantities as your hard disk approaches capacity (and no, the decrease in seek time is not a factor of bloat, it's a factor of the level of entropy in respect to block contiguousness).
"Search bar suggestions take longer to load"... again, the USER's fault for allowing a search bar to be installed (for example, when you install Sun's Java JRE or JDK, it offers to install the google toolbar). Not to mention that this is not considered bloat as (a) the USER has to choose to install the option and (b) once installed the feature does not grow in perpetuity. Sorry, wrong again.
"My computer gets slower with every drive you add"... your computer does... not mine. Again, see gpedit.msc to disable automatic statistical analysis/tracking of internal disks.
"Sometimes programs install themselves to context menus" ... solution: don't use said programs (I make it a point to not use programs that install unwanted/non-optional components... for example, any program that installs a Browser-Helper Object or BHO without my permission gets mercilessly killed immediately after install). Go to sysinternals.com (now owned and operated by Microsoft) and get yourself a copy of these four utilities: ProcessExplorer, AutoRuns, NTFileMon, and TCPView. These four utilities will help you to uninstall anything that is undesired and find any malware. Again, claiming that your computer spontaneously self-bloats without your interaction is both naive and a clear attempt at denying the user's negligence. Might you be referring to the WinRar context menu? You are allowed to uncheck the box for context menu items during install (which you probably neglected to look into, therefore YOU were the root-cause of any perceived bloat).
"How about programs that have background processes always running" ... like I said... the operating system does not spontaneously bloat-up... it takes YOU to help it. Either you surfed some pr0n that you shouldn't have, allowing security exploits in your browser to install malware and make your system part of a bot-net (a zombie that does the bidding of a remote hacker on some IRC channel). It's not Microsofts fault that you love you some dirty hairy pr0n, so much so that you're willing to visit websites infected with malware. Oh, and you were too busy with that pr0n to take some responsibility for the ensuing bloat (which you yourself caused) by installing some Anti-Malware software (which can't be considered as bloat either, because you installed it because you needed protection from the dirty pr0n-sites). Oh, were you referring to legitimate software that runs background tasks? Well, presumably you clicked the button to install said software too, so once again, we're back to YOU as the problem. Java did not install itself. You had a hand in it's install, even if you didn't explicitly install Java (that is, you installed product XYZ that required Java, so you indirectly installed Java).
"And why can't more registry items slow down windows?" I know you're being sarcastic, so I'll answer as such. Again, things don't get added to the registry spontaneously. Do this exercise... install Windows on a machine with a battery-backed power-supply, then put said workstation into a physical bank-vault (not connected to the Internet)... come back in a few hours... did the system spontaneously fill up the registry and hard disk? NO, because, sir, YOU ARE THE IDIOT! Systems don't spontaneously decide to get slower and more bloated... they need YOUR HELP FOR THAT!
"Storing it in memory takes..." ... blah blah blah
We've moved away from the realm of "self-bloat" and are now talking about raw memory operations and binary search heuristics? Bloat, in it's most canonical form, implies "data that increases in size". I neither see how memory operations or searching for binary data has anything to do with data growth. You clearly have no idea what you're talking about.
"*drops the mic*" ... because you are a first-year CIS student trying to argue with a post-doctoral professional (of whom you just called an idiot) that can run circles around you in practically any topic.