Why can't I place additional empty lots in Sims 4??

There are additional worlds that come with packs. Do you only have the base game?

No, you still can only use the premade lots already in the game, but there will be more of them (New Towns). There are discussions about this and you're not the only one that is sad about there being no "create a world"-tool. Most people think that it won't happen though. Sorry :/

Any updates on this? I think it's very strange, and unecessarily limiting the player.

No.

@daniellecyr I also wish there were more options for developing and build ing/placing lots we choose for our own 'customized' worlds. As @Phantomlover1717 mentioned, certain expansions do come with additional worlds, which will also be a combination of retail/public plus already built (filled and unfilled) residential homes, plus a sprinkling of empty lots. A few things to point out, in case you didn't realize this:

• You can evict families in the lots. They will then 'live' in the played/unplayed households, and they will still be 'townies' and go to businesses, public lots, etc... they just won't live in one of the houses. Then you can use or build on or move into those lots. (If you want to you can save the lot to your library before building over it, in case you may want to re-add it somewhere later). If you want make sure any particular sim household that you like but don't play doesn't get deleted, open it and save or just 'favorite' it in the households. Check to make sure but either of these actions should add them to your 'played households' and protect them from deletion/replacement by the game.
• You can edit, bulldoze, build on, or download a lot from the gallery to place on certain retail and public lots (the ones that highlight when you hover on them and allow you to enter in edit/build mode). (If you want to you can save the lot to your library before building over it, in case you may want to re-add it somewhere later).
• You probably know this, but most of the worlds are visited by/used by you and other community sims (played and unplayed). (The destination worlds are a bit different, I believe, but I only have one of those and have used it a limited amount, so I cannot give you a 100% accurate view on this part.
• I do a lot of combining households, and most of my households are 2-3 couples plus grandparent(s), kids, pets, or others you move into the household. This forces you to need a bigger home, but it provides a lot of support (financial, housework, social, etc.) for the household members, and it may allow you to clear out a few homes to do some of this?
• If most of the couples only have 1-3 kids, your lots won't fill up quite so fast. You can then fill the household with grandparents, a favorite community couple, etc. (as mentioned above).
• You can create a new game that features certain families, but doesn't include them all, and switch your play between your various game saves. Directions on this process: 
  https://youtu.be/zWuCxjTlJf0
  https://forums.thesims.com/en_us/discussion/807446/what-happens-if-i-click-new-game

Do you have any additional worlds besides what comes with the base game? If not, they seem to be 40%-50% off currently, and they is super helpful for enjoying and expanding your game! You can find the sales on this page: https://www.ea.com/en-gb/games/the-sims/the-sims-4/pc/store  ( @daniellecyr - you asked about this...)

I would most recommend the Seasons Expansion (but I don't think it comes with a new world). 'Get Famous' is great and adds the world 'De Sol Valley'. 'Get Together' adds the beautiful, quaint world 'Windenburg' - one of my favorite worlds to build in. I think 'Magnolia Promenade' came with 'Get to Work'. I love that pack, but that new world is pretty plain with only 3-4 lots. 'San Myshuno' comes with City Living, and 'Forgotten Hollow' comes with vampires. 'Brindleton Bay' has good 'personality' to the landscaping also and comes with 'Cats and Dogs'.

Hope something in all this is helpful for you. Sorry for your frustration. 

I was also wondering this.  I have several of the worlds, but some only have 3-4 lots in them and what looks like places I could put lots.  I would really love to place more lots into some of these worlds.

I would love the option to place Empty Lots, especially the larger 64by64 lots, since there are so few in the game already and sometimes I will find a great house/lots but it will be massive and it will only fit in that lot and then it wipes out the park. 

This game is * and needs way more lots to build on