Nexus doesn't. Moto X doesn't. Samsung does. Apple doesn't. LG does. It's not like the entire industry either does, or does not supply removable memory. And even those that do, who says it's easy? For awhile, I had a Sony Xperia Z Ultra. I sold it on eBay for a slight loss because I got tired of it. Quickly. Only had 16gb, but I didn't care, because it had that vaunted memory slot. Then I got the phone, and perhaps it's easier in Android 5.0, but on 4.X here's what I had to do to get things off of internal memory:
1) Insert a Micro SD card formatted with two partitions, one Ext4 for app storage and one FAT for data storage.
2) Root my phone.
3) Install a SU utility.
4) Purchase and install an app that would let me "link" app data to the SD card, since there was no built in way to move apps (I understand some phones have the capability, others don't).
5) Purchase and install FolderSync to handle application data moves, for apps that didn't support specifying a non-standard directory.
Then I had to live with the fact that I could no longer stream TiVo content to the phone, thanks to step #2 above.
So while my Sony had 16gb internal and 64gb external, and the performance on the external was actually pretty good, since I bought a quality, premium, fast SanDisk Ultra MicroSDXC card, and actually became terrific after installing and configuring a tuning app that let me change the caching settings and other parameters, I have an easier time today with my Moto X 32gb. Less storage, less headaches. And TiVo works again.
I understand some of the why (try to make casual pirating of apps harder by keeping them on a partition type that most people outside of Slashdot won't be able to read), but frankly it just wasn't worth it. I'm a techie. I had no trouble getting those steps figured out. I just don't want to do that. Then Lollipop was about to ship, right after I sold my phone, and I thought "I wonder if all those apps still work, or what the new process is". I just didn't want to go through with that. Kudos to Samsung for support SD slots. I hope moving apps is easier, but I hate their UI so I never consider their phones. Had an S3 once. Hated it. I'm sure TouchWiz is much better now, but I like Google's own interface best. So I had a Sony Xperia GPE and now a Moto X Pure edition.
Incidentally, Microsoft has figured it out better than Apple or Google and their phone OEMs. Got my kids a Lumia (hard to pass up for under $50 at their age when they just want to play the few games that exist for every major platform anyway). Just tick a box and all apps default to SD storage, except for an occasional app whose developer prohibits the behavior, in which case you're prompted and allowed to install it on internal memory.