Why is Photoshop by itself more expensive than the Lightroom + Photoshop combo?

The desktop version of Photoshop is the same in all cases for all Creative Cloud plans. The Photography plans include either the cloud-based Lightroom desktop app or Lightroom Classic, or both.

The biggest differences are that the single-app Photoshop plan doesn't include any version of Lightroom – however, it does give you the full version of Adobe Fresco as well as the premium version of Creative Cloud Express (both of which normally cost $9.99/mo. by themselves), plus more online cloud storage.

The Photoshop single-app plan also can be purchased month-to-month, whereas the Photography plans cannot.

This plan comparison table should make everything clearer.

Customers usually find one of the three Photography-related bundles to be of better value – but it definitely depends on your needs.

Another way to see it: Adobe offer standard plans for all single products $20/mo for an annual commitment or $30/mo for cancel without penalty. This is easy for buyers to understand, especially in business, though most businesses will want team or enterprise plans - and there is no Photography plan for teams. The Photography plan is for the price-sensitive consumer, and by pitching it low Adobe bring in millions of customers...

I'm assuming the photoshop version in this plan would have some features removed,

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Not true.  It's the same software in BOTH plans.  But Photography Plan is a 12th month commitment. 

The special photography plan is also somewhat like a 'loss leader' advertisement to get you into a grocery store to buy the items on sale, while hoping that you will decide to add other purchases... the photoshop in the special plan is exactly the same as the photoshop sold as a single product for a higher price

Because the Photography Plan is a deeply discounted bundle aimed at digital photographers. 

Creative Cloud Photography Plan -- approx $10/month for 12 months. Includes the very latest versions of these great products & services:
-- Photoshop CC and Photoshop on iPad
-- Lightroom on desktop, mobile and the web
-- Lightroom Classic
-- Portfolio website + hosting
-- Spark with premium features
-- Bridge
-- 20 GB cloud storage (upgradable to 1 TB).
https://www.adobe.com/creativecloud/photography.html

If you only needed Photoshop for a 1-2 month long project, you would need to buy the costlier monthly single app plan.  Different options for different types of users.