The reason that most new or rebuilt websites don’t show up very well on Google is simply because the vast majority of web designers are creative people, and not technical people. They know how to use a theme on WordPress, can adjust a pre-supplied theme on Squarespace or Wix, but they have no idea what it takes to help a website have impact on search engines. Unfortunately their quote often doesn’t state this, so you’re left assuming that they’ve also done something to get your site on Google.
Web designers love design. And that’s dangerous.

The first part that many web designers miss when it comes to search engine optimisation, is that many of their design elements, huge hero images, slideshows, background videos and the general “prettiness” is what really slows websites down. Most of the problem comes with super-large images, videos with even larger file sizes and over-use of design elements, especially in WordPress websites, that are delivered using heavy and bloated Javascript code. Javascript in itself isn’t a bad thing. It runs most of the cool stuff we have come to enjoy on the web. But when it’s used to present stuff that could just as easily be presented as text, you get in to trouble. And while no one is advocating for a boring internet full of nothing but text, if you want to rate highly on Google’s index, you need to be fast, lean and easy to use for everyone. Especially on mobile phones, and that brings me to the next point.
Web designers often design for their world, not ours.

Every web designer makes websites on laptop or desktop computers. Most web traffic these days come from mobile phones. Can you see the disconnect here? It’s actually really difficult to design a website on a mobile phone. But it is remarkably easily to design a mobile-first website on a computer. And I say, “mobile-first” rather than “mobile-friendly” because you’re either designing for where the traffic is coming from, or you’re not. If you’re a business that sells primarily to consumers, then they’re coming in overwhelmingly from mobiles. If you’re selling to businesses, then you may not have quite the same break-down of sources, but you’ll still see a significant number of people coming to you from mobile devices. When you design for something to be readable, fast and attractive on a mobile device, you’re also designing for it to be readable, fast and attractive on a desktop device. Just about every major web platform now is made to work really well on mobiles. WordPress page builders are great at it. Joomla page builders are much the same. Wix has an excellent mobile-oriented page building environment that actually lets you easily design a completely different layout on mobile to desktop. When web designers design on computers, they often forget to test on mobiles before they go live. So they can’t see that those giant footers, giant headers, over-sized images and slideshows don’t translate well at all to phones – and that sees those sites penalised heavily in Google where their indexing is done for mobile FIRST. And then for desktop. So if you’re not performing well on mobile, you’re not going to perform well on desktop either.
Web designers aren’t writers.

A third point is that web designers are not writers. While the principles of on-page SEO are simple to learn for someone with experience in copywriting, web designers are closer to being graphic designers. They work in a realm of aesthetics if they are more creatively geared. More technical web designers are more likely to be web “developers” who know just enough about design to get away with doing a similar look for every website they build. Writing effectively for search results is like writing effectively for people to read. You’re not stuffing “key words” in to as many sentences as possible. You’re not baffling people with techno-babble because you think that Google will be impressed. You write for the way that people want to read your content. Which means researching what people are searching for, what they want answered, what they need and how they want to find out about it. Few people want to read 3000 words of fluff to finally see the answer to their question. They want to know it in the first sentence if possible. Sure, you can fluff on after that, but just deliver people the answers they seek, in the way that they best understand it and you’ll go a long way to ranking with Google. Because web designers are creative, and see what they do as a form of art, so too, their words can go on, and on, and on. They will look for 15 ways to repeat the same sentence over and over with different words. And this is because they are trying to fill space to balance out their design between graphics and text. In the old web design world, this mattered. In the mobile world, it’s all in one vertical column, so it just doesn’t matter anymore.
Design & SEO should work together
There’s many more examples of how web designers just aren’t search engine optimisers. And often, SEO people aren’t web designers either. So it makes sense to work with both, preferable at the same time. After all, you need the speed and usability of the SEO person, but you also want a beautiful and attractive website.