Because the engineering mantra of designing something that's the minimum needed to do the job properly has been supplanted with a long-term strategic goal to attempt to sell more things to consumers by selling them devices that don't do everything they need out of the box.
Is this feature reduction or future proofing? I have a laptop with a SD slot and HDMI port that I've never used, except to only prove to myself that they worked, and not likely to use in the future. It also has USB-3 and DisplayPort outputs which with inexpensive adapters I can use to attach an SD card reader or HDMI cable. If given the option now I'd much rather buy a replacement that lacks a SD reader and HDMI port so that I can have a laptop that is just that much smaller, lighter, and cheaper.
I've had an iPod Touch for years that has seen daily use, and is now going into semi-retirement with my acquisition of an iPhone. That iPod had it's headphone jack damaged about a year ago but after an initial transition period I didn't miss it. I could still dock it with my truck stereo for music. When at home I could stream my music to an AirPort Expess, put in in a dock by my stereo, or just listen to it through the internal speaker. This is how I intend to use my iPhone now. What allowed me to keep that iPod working for me for so long was the ability to get audio and video from the dock port. I didn't need all kinds of ports and plugs on the iPod itself, I just bought the cables as I needed them. These cables and adapters included a composite A/V cable, component A/V cable, USB "card reader" adapter, and the car stereo adapter I mentioned earlier. An iPod with all of those ports on the device itself would have been huge.
This is a bit different with a laptop due to the inherent proportions of the format. I do remember many many people essentially laughing at Apple for not putting an optical drive in their laptop. Now we have people laughing at them for not having a SD slot. In the past I hated having to need adapters because they were exceedingly large and expensive, or so I perceived them to be, and it seemed I could never find the one I needed when I needed it. What has changed is the technology, adapters are smaller and cheaper now, and with the growth in the internet I have access to many competing suppliers trying to get me what I want when I want it.
Another change, perhaps just as important, is my perception. I have come to realize that no matter what two devices I have before me that I wish to connect I will need an adapter. We might not perceive this as an adapter but as a cable but every cable is effectively an adapter. Instead of thinking of this as having to buy another damned adapter I think of it as having to get a cable I would have had to get anyway but now I don't have to have a dozen ports on a computer where I'll only use half of them.
A joke among my friends was that USB stood for "useless serial bus" since when it was introduced there was nothing to plug into it. Now it's replaced nearly everything and I'm liking it. I don't need a laptop with a serial port, Ethernet port, flash card reader, modem port, Firewire port, parallel port, and DVI port like my old one did. When I pack my laptop I also pack the cables I need with the USB adapters attached. I treat the USB adapter and cable as a single unit, if it isn't a single physical unit already. While USB isn't quite "universal" it's close enough that I only need a couple other kinds of ports to plug into everything I need to get my work done.
Another thing that has changed with time is the weight bearing ability of my knees. Having all those ports on the laptop means weight that I must carry even if I just want to have a laptop with me to do a bit of work at a deli while eating. I'll still pack my bag with my laptop but all those adapters can be left behind.
So, yes, they do intend for people to come back for the cable they need. Any more I find complaints that a device doesn't have the precise port you require is like complaining your pants didn't come with a belt. Not everyone is going to agree on what kind of belt goes with those pants just like not everyone is going to agree on what kind of port goes on the computer. If the pants didn't come with belt loops then that is something else, since it prevents the use of any belt. So long as the computer or smart phone has a port to get the right cable I find it real hard to complain any more.