I assume the reason this wasn't added was to encourage use of the Contact form instead (given email addresses are hidden, there's flood protection and you can allow users to enable/disable it). However, clearly there are reasons for it being convenient to see the actual address.
(Do take time to check your permissions for user profile pages though, so you're not exposing people to spam.)
Here's a modified user.html.twig
, using classy as the base theme, with clickable address and a translatable string for the label:
<article{{ attributes.addClass('profile') }}>
{% if content %}
<div class="field field--name-email field--label-above">
<div class="field__label">{% trans %}Email{% endtrans %}</div>
<div class="field__item"><a href="mailto:{{ user.mail.value }}">{{ user.mail.value }}</a></div>
</div>
{{- content -}}
{% endif %}
</article>